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class="picker-header"> 117 <a href="#"> 118 <span class="collapsed-arrow">►</span><span class="expanded-arrow">▼</span> 119 Table of contents 120 </a> 121 122 <div class="picker"><div class="toc"><ul> 123<li><a href="#modules-ecmascript-modules">Modules: ECMAScript modules</a> 124<ul> 125<li><a href="#introduction">Introduction</a></li> 126<li><a href="#enabling">Enabling</a></li> 127<li><a href="#packages">Packages</a></li> 128<li><a href="#import-specifiers"><code>import</code> Specifiers</a> 129<ul> 130<li><a href="#terminology">Terminology</a></li> 131<li><a href="#mandatory-file-extensions">Mandatory file extensions</a></li> 132<li><a href="#urls">URLs</a> 133<ul> 134<li><a href="#file-urls"><code>file:</code> URLs</a></li> 135<li><a href="#data-imports"><code>data:</code> imports</a></li> 136<li><a href="#node-imports"><code>node:</code> imports</a></li> 137</ul> 138</li> 139</ul> 140</li> 141<li><span class="stability_1"><a href="#import-assertions">Import assertions</a></span></li> 142<li><a href="#builtin-modules">Builtin modules</a></li> 143<li><a href="#import-expressions"><code>import()</code> expressions</a></li> 144<li><a href="#importmeta"><code>import.meta</code></a> 145<ul> 146<li><a href="#importmetaurl"><code>import.meta.url</code></a></li> 147<li><span class="stability_1"><a href="#importmetaresolvespecifier-parent"><code>import.meta.resolve(specifier[, parent])</code></a></span></li> 148</ul> 149</li> 150<li><a href="#interoperability-with-commonjs">Interoperability with CommonJS</a> 151<ul> 152<li><a href="#import-statements"><code>import</code> statements</a></li> 153<li><a href="#require"><code>require</code></a></li> 154<li><a href="#commonjs-namespaces">CommonJS Namespaces</a></li> 155<li><a href="#differences-between-es-modules-and-commonjs">Differences between ES modules and CommonJS</a> 156<ul> 157<li><a href="#no-require-exports-or-moduleexports">No <code>require</code>, <code>exports</code>, or <code>module.exports</code></a></li> 158<li><a href="#no-__filename-or-__dirname">No <code>__filename</code> or <code>__dirname</code></a></li> 159<li><a href="#no-addon-loading">No Addon Loading</a></li> 160<li><a href="#no-requireresolve">No <code>require.resolve</code></a></li> 161<li><a href="#no-node_path">No <code>NODE_PATH</code></a></li> 162<li><a href="#no-requireextensions">No <code>require.extensions</code></a></li> 163<li><a href="#no-requirecache">No <code>require.cache</code></a></li> 164</ul> 165</li> 166</ul> 167</li> 168<li><span class="stability_1"><a href="#json-modules">JSON modules</a></span></li> 169<li><span class="stability_1"><a href="#wasm-modules">Wasm modules</a></span></li> 170<li><a href="#top-level-await">Top-level <code>await</code></a></li> 171<li><span class="stability_1"><a href="#https-and-http-imports">HTTPS and HTTP imports</a></span> 172<ul> 173<li><a href="#imports-are-limited-to-http1">Imports are limited to HTTP/1</a></li> 174<li><a href="#http-is-limited-to-loopback-addresses">HTTP is limited to loopback addresses</a></li> 175<li><a href="#authentication-is-never-sent-to-the-destination-server">Authentication is never sent to the destination server.</a></li> 176<li><a href="#cors-is-never-checked-on-the-destination-server">CORS is never checked on the destination server</a></li> 177<li><a href="#cannot-load-non-network-dependencies">Cannot load non-network dependencies</a></li> 178<li><a href="#network-based-loading-is-not-enabled-by-default">Network-based loading is not enabled by default</a></li> 179</ul> 180</li> 181<li><span class="stability_1"><a href="#loaders">Loaders</a></span> 182<ul> 183<li><a href="#hooks">Hooks</a> 184<ul> 185<li><a href="#resolvespecifier-context-nextresolve"><code>resolve(specifier, context, nextResolve)</code></a></li> 186<li><a href="#loadurl-context-nextload"><code>load(url, context, nextLoad)</code></a></li> 187<li><a href="#globalpreload"><code>globalPreload()</code></a></li> 188</ul> 189</li> 190<li><a href="#examples">Examples</a> 191<ul> 192<li><a href="#https-loader">HTTPS loader</a></li> 193<li><a href="#transpiler-loader">Transpiler loader</a></li> 194<li><a href="#import-map-loader">"import map" loader</a></li> 195</ul> 196</li> 197</ul> 198</li> 199<li><a href="#resolution-and-loading-algorithm">Resolution and loading algorithm</a> 200<ul> 201<li><a href="#features">Features</a></li> 202<li><a href="#resolution-algorithm">Resolution algorithm</a></li> 203<li><a href="#resolution-algorithm-specification">Resolution Algorithm Specification</a></li> 204<li><span class="stability_1"><a href="#customizing-esm-specifier-resolution-algorithm">Customizing ESM specifier resolution algorithm</a></span></li> 205</ul> 206</li> 207</ul> 208</li> 209</ul></div></div> 210 </li> 211 212 213 <li class="picker-header"> 214 <a href="#"> 215 <span class="collapsed-arrow">►</span><span class="expanded-arrow">▼</span> 216 Index 217 </a> 218 219 <div class="picker"><ul> 220<li><a href="documentation.html" class="nav-documentation">About this documentation</a></li> 221<li><a href="synopsis.html" class="nav-synopsis">Usage and example</a></li> 222 223 <li> 224 <a href="index.html">Index</a> 225 </li> 226 </ul> 227 228<hr class="line"> 229<ul> 230<li><a href="assert.html" class="nav-assert">Assertion testing</a></li> 231<li><a href="async_context.html" class="nav-async_context">Asynchronous context tracking</a></li> 232<li><a href="async_hooks.html" class="nav-async_hooks">Async hooks</a></li> 233<li><a href="buffer.html" class="nav-buffer">Buffer</a></li> 234<li><a href="addons.html" class="nav-addons">C++ addons</a></li> 235<li><a href="n-api.html" class="nav-n-api">C/C++ addons with Node-API</a></li> 236<li><a href="embedding.html" class="nav-embedding">C++ embedder API</a></li> 237<li><a href="child_process.html" class="nav-child_process">Child processes</a></li> 238<li><a href="cluster.html" class="nav-cluster">Cluster</a></li> 239<li><a 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class="nav-http2">HTTP/2</a></li> 254<li><a href="https.html" class="nav-https">HTTPS</a></li> 255<li><a href="inspector.html" class="nav-inspector">Inspector</a></li> 256<li><a href="intl.html" class="nav-intl">Internationalization</a></li> 257<li><a href="modules.html" class="nav-modules">Modules: CommonJS modules</a></li> 258<li><a href="esm.html" class="nav-esm active">Modules: ECMAScript modules</a></li> 259<li><a href="module.html" class="nav-module">Modules: <code>node:module</code> API</a></li> 260<li><a href="packages.html" class="nav-packages">Modules: Packages</a></li> 261<li><a href="net.html" class="nav-net">Net</a></li> 262<li><a href="os.html" class="nav-os">OS</a></li> 263<li><a href="path.html" class="nav-path">Path</a></li> 264<li><a href="perf_hooks.html" class="nav-perf_hooks">Performance hooks</a></li> 265<li><a href="permissions.html" class="nav-permissions">Permissions</a></li> 266<li><a href="process.html" class="nav-process">Process</a></li> 267<li><a href="punycode.html" class="nav-punycode">Punycode</a></li> 268<li><a href="querystring.html" class="nav-querystring">Query strings</a></li> 269<li><a href="readline.html" class="nav-readline">Readline</a></li> 270<li><a href="repl.html" class="nav-repl">REPL</a></li> 271<li><a href="report.html" class="nav-report">Report</a></li> 272<li><a href="single-executable-applications.html" class="nav-single-executable-applications">Single executable applications</a></li> 273<li><a href="stream.html" class="nav-stream">Stream</a></li> 274<li><a href="string_decoder.html" class="nav-string_decoder">String decoder</a></li> 275<li><a href="test.html" class="nav-test">Test runner</a></li> 276<li><a href="timers.html" class="nav-timers">Timers</a></li> 277<li><a href="tls.html" class="nav-tls">TLS/SSL</a></li> 278<li><a href="tracing.html" class="nav-tracing">Trace events</a></li> 279<li><a href="tty.html" class="nav-tty">TTY</a></li> 280<li><a href="dgram.html" class="nav-dgram">UDP/datagram</a></li> 281<li><a href="url.html" class="nav-url">URL</a></li> 282<li><a href="util.html" class="nav-util">Utilities</a></li> 283<li><a href="v8.html" class="nav-v8">V8</a></li> 284<li><a href="vm.html" class="nav-vm">VM</a></li> 285<li><a href="wasi.html" class="nav-wasi">WASI</a></li> 286<li><a href="webcrypto.html" class="nav-webcrypto">Web Crypto API</a></li> 287<li><a href="webstreams.html" class="nav-webstreams">Web Streams API</a></li> 288<li><a href="worker_threads.html" class="nav-worker_threads">Worker threads</a></li> 289<li><a href="zlib.html" class="nav-zlib">Zlib</a></li> 290</ul> 291<hr class="line"> 292<ul> 293<li><a href="https://github.com/nodejs/node" class="nav-https-github-com-nodejs-node">Code repository and issue tracker</a></li> 294</ul></div> 295 </li> 296 297 298 <li class="picker-header"> 299 <a href="#"> 300 <span class="collapsed-arrow">►</span><span class="expanded-arrow">▼</span> 301 Other versions 302 </a> 303 <div class="picker"><ol id="alt-docs"><li><a href="https://nodejs.org/docs/latest-v20.x/api/esm.html">20.x</a></li> 304<li><a href="https://nodejs.org/docs/latest-v19.x/api/esm.html">19.x</a></li> 305<li><a href="https://nodejs.org/docs/latest-v18.x/api/esm.html">18.x <b>LTS</b></a></li> 306<li><a href="https://nodejs.org/docs/latest-v17.x/api/esm.html">17.x</a></li> 307<li><a href="https://nodejs.org/docs/latest-v16.x/api/esm.html">16.x</a></li> 308<li><a href="https://nodejs.org/docs/latest-v15.x/api/esm.html">15.x</a></li> 309<li><a href="https://nodejs.org/docs/latest-v14.x/api/esm.html">14.x</a></li> 310<li><a href="https://nodejs.org/docs/latest-v13.x/api/esm.html">13.x</a></li> 311<li><a href="https://nodejs.org/docs/latest-v12.x/api/esm.html">12.x</a></li> 312<li><a href="https://nodejs.org/docs/latest-v11.x/api/esm.html">11.x</a></li> 313<li><a href="https://nodejs.org/docs/latest-v10.x/api/esm.html">10.x</a></li> 314<li><a href="https://nodejs.org/docs/latest-v9.x/api/esm.html">9.x</a></li> 315<li><a href="https://nodejs.org/docs/latest-v8.x/api/esm.html">8.x</a></li></ol></div> 316 </li> 317 318 <li class="picker-header"> 319 <a href="#"> 320 <span class="collapsed-arrow">►</span><span class="expanded-arrow">▼</span> 321 Options 322 </a> 323 324 <div class="picker"> 325 <ul> 326 <li> 327 <a href="all.html">View on single page</a> 328 </li> 329 <li> 330 <a href="esm.json">View as JSON</a> 331 </li> 332 <li class="edit_on_github"><a href="https://github.com/nodejs/node/edit/main/doc/api/esm.md">Edit on GitHub</a></li> 333 </ul> 334 </div> 335 </li> 336 </ul> 337 </div> 338 <hr> 339 </header> 340 341 <details id="toc" open><summary>Table of contents</summary><ul> 342<li><a href="#modules-ecmascript-modules">Modules: ECMAScript modules</a> 343<ul> 344<li><a href="#introduction">Introduction</a></li> 345<li><a href="#enabling">Enabling</a></li> 346<li><a href="#packages">Packages</a></li> 347<li><a href="#import-specifiers"><code>import</code> Specifiers</a> 348<ul> 349<li><a href="#terminology">Terminology</a></li> 350<li><a href="#mandatory-file-extensions">Mandatory file extensions</a></li> 351<li><a href="#urls">URLs</a> 352<ul> 353<li><a href="#file-urls"><code>file:</code> URLs</a></li> 354<li><a href="#data-imports"><code>data:</code> imports</a></li> 355<li><a href="#node-imports"><code>node:</code> imports</a></li> 356</ul> 357</li> 358</ul> 359</li> 360<li><span class="stability_1"><a href="#import-assertions">Import assertions</a></span></li> 361<li><a href="#builtin-modules">Builtin modules</a></li> 362<li><a href="#import-expressions"><code>import()</code> expressions</a></li> 363<li><a href="#importmeta"><code>import.meta</code></a> 364<ul> 365<li><a href="#importmetaurl"><code>import.meta.url</code></a></li> 366<li><span class="stability_1"><a href="#importmetaresolvespecifier-parent"><code>import.meta.resolve(specifier[, parent])</code></a></span></li> 367</ul> 368</li> 369<li><a href="#interoperability-with-commonjs">Interoperability with CommonJS</a> 370<ul> 371<li><a href="#import-statements"><code>import</code> statements</a></li> 372<li><a href="#require"><code>require</code></a></li> 373<li><a href="#commonjs-namespaces">CommonJS Namespaces</a></li> 374<li><a href="#differences-between-es-modules-and-commonjs">Differences between ES modules and CommonJS</a> 375<ul> 376<li><a href="#no-require-exports-or-moduleexports">No <code>require</code>, <code>exports</code>, or <code>module.exports</code></a></li> 377<li><a href="#no-__filename-or-__dirname">No <code>__filename</code> or <code>__dirname</code></a></li> 378<li><a href="#no-addon-loading">No Addon Loading</a></li> 379<li><a href="#no-requireresolve">No <code>require.resolve</code></a></li> 380<li><a href="#no-node_path">No <code>NODE_PATH</code></a></li> 381<li><a href="#no-requireextensions">No <code>require.extensions</code></a></li> 382<li><a href="#no-requirecache">No <code>require.cache</code></a></li> 383</ul> 384</li> 385</ul> 386</li> 387<li><span class="stability_1"><a href="#json-modules">JSON modules</a></span></li> 388<li><span class="stability_1"><a href="#wasm-modules">Wasm modules</a></span></li> 389<li><a href="#top-level-await">Top-level <code>await</code></a></li> 390<li><span class="stability_1"><a href="#https-and-http-imports">HTTPS and HTTP imports</a></span> 391<ul> 392<li><a href="#imports-are-limited-to-http1">Imports are limited to HTTP/1</a></li> 393<li><a href="#http-is-limited-to-loopback-addresses">HTTP is limited to loopback addresses</a></li> 394<li><a href="#authentication-is-never-sent-to-the-destination-server">Authentication is never sent to the destination server.</a></li> 395<li><a href="#cors-is-never-checked-on-the-destination-server">CORS is never checked on the destination server</a></li> 396<li><a href="#cannot-load-non-network-dependencies">Cannot load non-network dependencies</a></li> 397<li><a href="#network-based-loading-is-not-enabled-by-default">Network-based loading is not enabled by default</a></li> 398</ul> 399</li> 400<li><span class="stability_1"><a href="#loaders">Loaders</a></span> 401<ul> 402<li><a href="#hooks">Hooks</a> 403<ul> 404<li><a href="#resolvespecifier-context-nextresolve"><code>resolve(specifier, context, nextResolve)</code></a></li> 405<li><a href="#loadurl-context-nextload"><code>load(url, context, nextLoad)</code></a></li> 406<li><a href="#globalpreload"><code>globalPreload()</code></a></li> 407</ul> 408</li> 409<li><a href="#examples">Examples</a> 410<ul> 411<li><a href="#https-loader">HTTPS loader</a></li> 412<li><a href="#transpiler-loader">Transpiler loader</a></li> 413<li><a href="#import-map-loader">"import map" loader</a></li> 414</ul> 415</li> 416</ul> 417</li> 418<li><a href="#resolution-and-loading-algorithm">Resolution and loading algorithm</a> 419<ul> 420<li><a href="#features">Features</a></li> 421<li><a href="#resolution-algorithm">Resolution algorithm</a></li> 422<li><a href="#resolution-algorithm-specification">Resolution Algorithm Specification</a></li> 423<li><span class="stability_1"><a href="#customizing-esm-specifier-resolution-algorithm">Customizing ESM specifier resolution algorithm</a></span></li> 424</ul> 425</li> 426</ul> 427</li> 428</ul></details> 429 430 <div id="apicontent"> 431 <h2>Modules: ECMAScript modules<span><a class="mark" href="#modules-ecmascript-modules" id="modules-ecmascript-modules">#</a></span><a aria-hidden="true" class="legacy" id="esm_modules_ecmascript_modules"></a></h2> 432 433 434<div class="api_metadata"> 435<details class="changelog"><summary>History</summary> 436<table> 437<tbody><tr><th>Version</th><th>Changes</th></tr> 438<tr><td>v18.6.0</td> 439<td><p>Add support for chaining loaders.</p></td></tr> 440<tr><td>v17.1.0, v16.14.0</td> 441<td><p>Add support for import assertions.</p></td></tr> 442<tr><td>v17.0.0, v16.12.0</td> 443<td><p>Consolidate loader hooks, removed <code>getFormat</code>, <code>getSource</code>, <code>transformSource</code>, and <code>getGlobalPreloadCode</code> hooks added <code>load</code> and <code>globalPreload</code> hooks allowed returning <code>format</code> from either <code>resolve</code> or <code>load</code> hooks.</p></td></tr> 444<tr><td>v14.8.0</td> 445<td><p>Unflag Top-Level Await.</p></td></tr> 446<tr><td>v15.3.0, v14.17.0, v12.22.0</td> 447<td><p>Stabilize modules implementation.</p></td></tr> 448<tr><td>v14.13.0, v12.20.0</td> 449<td><p>Support for detection of CommonJS named exports.</p></td></tr> 450<tr><td>v14.0.0, v13.14.0, v12.20.0</td> 451<td><p>Remove experimental modules warning.</p></td></tr> 452<tr><td>v13.2.0, v12.17.0</td> 453<td><p>Loading ECMAScript modules no longer requires a command-line flag.</p></td></tr> 454<tr><td>v12.0.0</td> 455<td><p>Add support for ES modules using <code>.js</code> file extension via <code>package.json</code> <code>"type"</code> field.</p></td></tr> 456<tr><td>v8.5.0</td> 457<td><p><span>Added in: v8.5.0</span></p></td></tr> 458</tbody></table> 459</details> 460</div> 461<p></p><div class="api_stability api_stability_2"><a href="documentation.html#stability-index">Stability: 2</a> - Stable</div><p></p> 462<section><h3>Introduction<span><a class="mark" href="#introduction" id="introduction">#</a></span><a aria-hidden="true" class="legacy" id="esm_introduction"></a></h3> 463 464<p>ECMAScript modules are <a href="https://tc39.github.io/ecma262/#sec-modules">the official standard format</a> to package JavaScript 465code for reuse. Modules are defined using a variety of <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/import"><code>import</code></a> and 466<a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/export"><code>export</code></a> statements.</p> 467<p>The following example of an ES module exports a function:</p> 468<pre><code class="language-js"><span class="hljs-comment">// addTwo.mjs</span> 469<span class="hljs-keyword">function</span> <span class="hljs-title function_">addTwo</span>(<span class="hljs-params">num</span>) { 470 <span class="hljs-keyword">return</span> num + <span class="hljs-number">2</span>; 471} 472 473<span class="hljs-keyword">export</span> { addTwo };</code> <button class="copy-button">copy</button></pre> 474<p>The following example of an ES module imports the function from <code>addTwo.mjs</code>:</p> 475<pre><code class="language-js"><span class="hljs-comment">// app.mjs</span> 476<span class="hljs-keyword">import</span> { addTwo } <span class="hljs-keyword">from</span> <span class="hljs-string">'./addTwo.mjs'</span>; 477 478<span class="hljs-comment">// Prints: 6</span> 479<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-title function_">addTwo</span>(<span class="hljs-number">4</span>));</code> <button class="copy-button">copy</button></pre> 480<p>Node.js fully supports ECMAScript modules as they are currently specified and 481provides interoperability between them and its original module format, 482<a href="modules.html">CommonJS</a>.</p> 483<!-- Anchors to make sure old links find a target --> 484<p><i id="esm_package_json_type_field"></i><i id="esm_package_scope_and_file_extensions"></i><i id="esm_input_type_flag"></i></p> 485</section><section><h3>Enabling<span><a class="mark" href="#enabling" id="enabling">#</a></span><a aria-hidden="true" class="legacy" id="esm_enabling"></a></h3> 486 487<p>Node.js has two module systems: <a href="modules.html">CommonJS</a> modules and ECMAScript modules.</p> 488<p>Authors can tell Node.js to use the ECMAScript modules loader 489via the <code>.mjs</code> file extension, the <code>package.json</code> <a href="packages.html#type"><code>"type"</code></a> field, or the 490<a href="cli.html#--input-typetype"><code>--input-type</code></a> flag. Outside of those cases, Node.js will use the CommonJS 491module loader. See <a href="packages.html#determining-module-system">Determining module system</a> for more details.</p> 492<!-- Anchors to make sure old links find a target --> 493<p><i id="esm_package_entry_points"></i><i id="esm_main_entry_point_export"></i><i id="esm_subpath_exports"></i><i id="esm_package_exports_fallbacks"></i><i id="esm_exports_sugar"></i><i id="esm_conditional_exports"></i><i id="esm_nested_conditions"></i><i id="esm_self_referencing_a_package_using_its_name"></i><i id="esm_internal_package_imports"></i><i id="esm_dual_commonjs_es_module_packages"></i><i id="esm_dual_package_hazard"></i><i id="esm_writing_dual_packages_while_avoiding_or_minimizing_hazards"></i><i id="esm_approach_1_use_an_es_module_wrapper"></i><i id="esm_approach_2_isolate_state"></i></p> 494</section><section><h3>Packages<span><a class="mark" href="#packages" id="packages">#</a></span><a aria-hidden="true" class="legacy" id="esm_packages"></a></h3> 495<p>This section was moved to <a href="packages.html">Modules: Packages</a>.</p> 496</section><section><h3><code>import</code> Specifiers<span><a class="mark" href="#import-specifiers" id="import-specifiers">#</a></span><a aria-hidden="true" class="legacy" id="esm_import_specifiers"></a></h3> 497<h4>Terminology<span><a class="mark" href="#terminology" id="terminology">#</a></span><a aria-hidden="true" class="legacy" id="esm_terminology"></a></h4> 498<p>The <em>specifier</em> of an <code>import</code> statement is the string after the <code>from</code> keyword, 499e.g. <code>'node:path'</code> in <code>import { sep } from 'node:path'</code>. Specifiers are also 500used in <code>export from</code> statements, and as the argument to an <code>import()</code> 501expression.</p> 502<p>There are three types of specifiers:</p> 503<ul> 504<li> 505<p><em>Relative specifiers</em> like <code>'./startup.js'</code> or <code>'../config.mjs'</code>. They refer 506to a path relative to the location of the importing file. <em>The file extension 507is always necessary for these.</em></p> 508</li> 509<li> 510<p><em>Bare specifiers</em> like <code>'some-package'</code> or <code>'some-package/shuffle'</code>. They can 511refer to the main entry point of a package by the package name, or a 512specific feature module within a package prefixed by the package name as per 513the examples respectively. <em>Including the file extension is only necessary 514for packages without an <a href="packages.html#exports"><code>"exports"</code></a> field.</em></p> 515</li> 516<li> 517<p><em>Absolute specifiers</em> like <code>'file:///opt/nodejs/config.js'</code>. They refer 518directly and explicitly to a full path.</p> 519</li> 520</ul> 521<p>Bare specifier resolutions are handled by the <a href="#resolution-algorithm-specification">Node.js module 522resolution and loading algorithm</a>. 523All other specifier resolutions are always only resolved with 524the standard relative <a href="https://url.spec.whatwg.org/">URL</a> resolution semantics.</p> 525<p>Like in CommonJS, module files within packages can be accessed by appending a 526path to the package name unless the package's <a href="packages.html#nodejs-packagejson-field-definitions"><code>package.json</code></a> contains an 527<a href="packages.html#exports"><code>"exports"</code></a> field, in which case files within packages can only be accessed 528via the paths defined in <a href="packages.html#exports"><code>"exports"</code></a>.</p> 529<p>For details on these package resolution rules that apply to bare specifiers in 530the Node.js module resolution, see the <a href="packages.html">packages documentation</a>.</p> 531<h4>Mandatory file extensions<span><a class="mark" href="#mandatory-file-extensions" id="mandatory-file-extensions">#</a></span><a aria-hidden="true" class="legacy" id="esm_mandatory_file_extensions"></a></h4> 532<p>A file extension must be provided when using the <code>import</code> keyword to resolve 533relative or absolute specifiers. Directory indexes (e.g. <code>'./startup/index.js'</code>) 534must also be fully specified.</p> 535<p>This behavior matches how <code>import</code> behaves in browser environments, assuming a 536typically configured server.</p> 537<h4>URLs<span><a class="mark" href="#urls" id="urls">#</a></span><a aria-hidden="true" class="legacy" id="esm_urls"></a></h4> 538<p>ES modules are resolved and cached as URLs. This means that special characters 539must be <a href="url.html#percent-encoding-in-urls">percent-encoded</a>, such as <code>#</code> with <code>%23</code> and <code>?</code> with <code>%3F</code>.</p> 540<p><code>file:</code>, <code>node:</code>, and <code>data:</code> URL schemes are supported. A specifier like 541<code>'https://example.com/app.js'</code> is not supported natively in Node.js unless using 542a <a href="#https-loader">custom HTTPS loader</a>.</p> 543<h5><code>file:</code> URLs<span><a class="mark" href="#file-urls" id="file-urls">#</a></span><a aria-hidden="true" class="legacy" id="esm_file_urls"></a></h5> 544<p>Modules are loaded multiple times if the <code>import</code> specifier used to resolve 545them has a different query or fragment.</p> 546<pre><code class="language-js"><span class="hljs-keyword">import</span> <span class="hljs-string">'./foo.mjs?query=1'</span>; <span class="hljs-comment">// loads ./foo.mjs with query of "?query=1"</span> 547<span class="hljs-keyword">import</span> <span class="hljs-string">'./foo.mjs?query=2'</span>; <span class="hljs-comment">// loads ./foo.mjs with query of "?query=2"</span></code> <button class="copy-button">copy</button></pre> 548<p>The volume root may be referenced via <code>/</code>, <code>//</code>, or <code>file:///</code>. Given the 549differences between <a href="https://url.spec.whatwg.org/">URL</a> and path resolution (such as percent encoding 550details), it is recommended to use <a href="url.html#urlpathtofileurlpath">url.pathToFileURL</a> when importing a path.</p> 551<h5><code>data:</code> imports<span><a class="mark" href="#data-imports" id="data-imports">#</a></span><a aria-hidden="true" class="legacy" id="esm_data_imports"></a></h5> 552<div class="api_metadata"> 553<span>Added in: v12.10.0</span> 554</div> 555<p><a href="https://developer.mozilla.org/en-US/docs/Web/HTTP/Basics_of_HTTP/Data_URIs"><code>data:</code> URLs</a> are supported for importing with the following MIME types:</p> 556<ul> 557<li><code>text/javascript</code> for ES modules</li> 558<li><code>application/json</code> for JSON</li> 559<li><code>application/wasm</code> for Wasm</li> 560</ul> 561<pre><code class="language-js"><span class="hljs-keyword">import</span> <span class="hljs-string">'data:text/javascript,console.log("hello!");'</span>; 562<span class="hljs-keyword">import</span> _ <span class="hljs-keyword">from</span> <span class="hljs-string">'data:application/json,"world!"'</span> assert { <span class="hljs-attr">type</span>: <span class="hljs-string">'json'</span> };</code> <button class="copy-button">copy</button></pre> 563<p><code>data:</code> URLs only resolve <a href="#terminology">bare specifiers</a> for builtin modules 564and <a href="#terminology">absolute specifiers</a>. Resolving 565<a href="#terminology">relative specifiers</a> does not work because <code>data:</code> is not a 566<a href="https://url.spec.whatwg.org/#special-scheme">special scheme</a>. For example, attempting to load <code>./foo</code> 567from <code>data:text/javascript,import "./foo";</code> fails to resolve because there 568is no concept of relative resolution for <code>data:</code> URLs.</p> 569<h5><code>node:</code> imports<span><a class="mark" href="#node-imports" id="node-imports">#</a></span><a aria-hidden="true" class="legacy" id="esm_node_imports"></a></h5> 570<div class="api_metadata"> 571<details class="changelog"><summary>History</summary> 572<table> 573<tbody><tr><th>Version</th><th>Changes</th></tr> 574<tr><td>v16.0.0, v14.18.0</td> 575<td><p>Added <code>node:</code> import support to <code>require(...)</code>.</p></td></tr> 576<tr><td>v14.13.1, v12.20.0</td> 577<td><p><span>Added in: v14.13.1, v12.20.0</span></p></td></tr> 578</tbody></table> 579</details> 580</div> 581<p><code>node:</code> URLs are supported as an alternative means to load Node.js builtin 582modules. This URL scheme allows for builtin modules to be referenced by valid 583absolute URL strings.</p> 584<pre><code class="language-js"><span class="hljs-keyword">import</span> fs <span class="hljs-keyword">from</span> <span class="hljs-string">'node:fs/promises'</span>;</code> <button class="copy-button">copy</button></pre> 585</section><section><h3>Import assertions<span><a class="mark" href="#import-assertions" id="import-assertions">#</a></span><a aria-hidden="true" class="legacy" id="esm_import_assertions"></a></h3> 586<div class="api_metadata"> 587<span>Added in: v17.1.0, v16.14.0</span> 588</div> 589<p></p><div class="api_stability api_stability_1"><a href="documentation.html#stability-index">Stability: 1</a> - Experimental</div><p></p> 590<p>The <a href="https://github.com/tc39/proposal-import-assertions">Import Assertions proposal</a> adds an inline syntax for module import 591statements to pass on more information alongside the module specifier.</p> 592<pre><code class="language-js"><span class="hljs-keyword">import</span> fooData <span class="hljs-keyword">from</span> <span class="hljs-string">'./foo.json'</span> assert { <span class="hljs-attr">type</span>: <span class="hljs-string">'json'</span> }; 593 594<span class="hljs-keyword">const</span> { <span class="hljs-attr">default</span>: barData } = 595 <span class="hljs-keyword">await</span> <span class="hljs-keyword">import</span>(<span class="hljs-string">'./bar.json'</span>, { <span class="hljs-attr">assert</span>: { <span class="hljs-attr">type</span>: <span class="hljs-string">'json'</span> } });</code> <button class="copy-button">copy</button></pre> 596<p>Node.js supports the following <code>type</code> values, for which the assertion is 597mandatory:</p> 598 599 600 601 602 603 604 605 606 607 608 609 610 611<table><thead><tr><th>Assertion <code>type</code></th><th>Needed for</th></tr></thead><tbody><tr><td><code>'json'</code></td><td><a href="#json-modules">JSON modules</a></td></tr></tbody></table> 612</section><section><h3>Builtin modules<span><a class="mark" href="#builtin-modules" id="builtin-modules">#</a></span><a aria-hidden="true" class="legacy" id="esm_builtin_modules"></a></h3> 613<p><a href="modules.html#core-modules">Core modules</a> provide named exports of their public API. A 614default export is also provided which is the value of the CommonJS exports. 615The default export can be used for, among other things, modifying the named 616exports. Named exports of builtin modules are updated only by calling 617<a href="module.html#modulesyncbuiltinesmexports"><code>module.syncBuiltinESMExports()</code></a>.</p> 618<pre><code class="language-js"><span class="hljs-keyword">import</span> <span class="hljs-title class_">EventEmitter</span> <span class="hljs-keyword">from</span> <span class="hljs-string">'node:events'</span>; 619<span class="hljs-keyword">const</span> e = <span class="hljs-keyword">new</span> <span class="hljs-title class_">EventEmitter</span>();</code> <button class="copy-button">copy</button></pre> 620<pre><code class="language-js"><span class="hljs-keyword">import</span> { readFile } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:fs'</span>; 621<span class="hljs-title function_">readFile</span>(<span class="hljs-string">'./foo.txt'</span>, <span class="hljs-function">(<span class="hljs-params">err, source</span>) =></span> { 622 <span class="hljs-keyword">if</span> (err) { 623 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">error</span>(err); 624 } <span class="hljs-keyword">else</span> { 625 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(source); 626 } 627});</code> <button class="copy-button">copy</button></pre> 628<pre><code class="language-js"><span class="hljs-keyword">import</span> fs, { readFileSync } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:fs'</span>; 629<span class="hljs-keyword">import</span> { syncBuiltinESMExports } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:module'</span>; 630<span class="hljs-keyword">import</span> { <span class="hljs-title class_">Buffer</span> } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:buffer'</span>; 631 632fs.<span class="hljs-property">readFileSync</span> = <span class="hljs-function">() =></span> <span class="hljs-title class_">Buffer</span>.<span class="hljs-title function_">from</span>(<span class="hljs-string">'Hello, ESM'</span>); 633<span class="hljs-title function_">syncBuiltinESMExports</span>(); 634 635fs.<span class="hljs-property">readFileSync</span> === readFileSync;</code> <button class="copy-button">copy</button></pre> 636</section><section><h3><code>import()</code> expressions<span><a class="mark" href="#import-expressions" id="import-expressions">#</a></span><a aria-hidden="true" class="legacy" id="esm_import_expressions"></a></h3> 637<p><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/import">Dynamic <code>import()</code></a> is supported in both CommonJS and ES modules. In CommonJS 638modules it can be used to load ES modules.</p> 639</section><section><h3><code>import.meta</code><span><a class="mark" href="#importmeta" id="importmeta">#</a></span><a aria-hidden="true" class="legacy" id="esm_import_meta"></a></h3> 640<ul> 641<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a></li> 642</ul> 643<p>The <code>import.meta</code> meta property is an <code>Object</code> that contains the following 644properties.</p> 645<h4><code>import.meta.url</code><span><a class="mark" href="#importmetaurl" id="importmetaurl">#</a></span><a aria-hidden="true" class="legacy" id="esm_import_meta_url"></a></h4> 646<ul> 647<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> The absolute <code>file:</code> URL of the module.</li> 648</ul> 649<p>This is defined exactly the same as it is in browsers providing the URL of the 650current module file.</p> 651<p>This enables useful patterns such as relative file loading:</p> 652<pre><code class="language-js"><span class="hljs-keyword">import</span> { readFileSync } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:fs'</span>; 653<span class="hljs-keyword">const</span> buffer = <span class="hljs-title function_">readFileSync</span>(<span class="hljs-keyword">new</span> <span class="hljs-title function_">URL</span>(<span class="hljs-string">'./data.proto'</span>, <span class="hljs-keyword">import</span>.<span class="hljs-property">meta</span>.<span class="hljs-property">url</span>));</code> <button class="copy-button">copy</button></pre> 654<h4><code>import.meta.resolve(specifier[, parent])</code><span><a class="mark" href="#importmetaresolvespecifier-parent" id="importmetaresolvespecifier-parent">#</a></span><a aria-hidden="true" class="legacy" id="esm_import_meta_resolve_specifier_parent"></a></h4> 655<!-- 656added: 657 - v13.9.0 658 - v12.16.2 659changes: 660 - version: 661 - v16.2.0 662 - v14.18.0 663 pr-url: https://github.com/nodejs/node/pull/38587 664 description: Add support for WHATWG `URL` object to `parentURL` parameter. 665--> 666<p></p><div class="api_stability api_stability_1"><a href="documentation.html#stability-index">Stability: 1</a> - Experimental</div><p></p> 667<p>This feature is only available with the <code>--experimental-import-meta-resolve</code> 668command flag enabled.</p> 669<ul> 670<li><code>specifier</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> The module specifier to resolve relative to <code>parent</code>.</li> 671<li><code>parent</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> | <a href="url.html#the-whatwg-url-api" class="type"><URL></a> The absolute parent module URL to resolve from. If none 672is specified, the value of <code>import.meta.url</code> is used as the default.</li> 673<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise" class="type"><Promise></a></li> 674</ul> 675<p>Provides a module-relative resolution function scoped to each module, returning 676the URL string.</p> 677<!-- eslint-skip --> 678<pre><code class="language-js"><span class="hljs-keyword">const</span> dependencyAsset = <span class="hljs-keyword">await</span> <span class="hljs-keyword">import</span>.<span class="hljs-property">meta</span>.<span class="hljs-title function_">resolve</span>(<span class="hljs-string">'component-lib/asset.css'</span>);</code> <button class="copy-button">copy</button></pre> 679<p><code>import.meta.resolve</code> also accepts a second argument which is the parent module 680from which to resolve from:</p> 681<!-- eslint-skip --> 682<pre><code class="language-js"><span class="hljs-keyword">await</span> <span class="hljs-keyword">import</span>.<span class="hljs-property">meta</span>.<span class="hljs-title function_">resolve</span>(<span class="hljs-string">'./dep'</span>, <span class="hljs-keyword">import</span>.<span class="hljs-property">meta</span>.<span class="hljs-property">url</span>);</code> <button class="copy-button">copy</button></pre> 683<p>This function is asynchronous because the ES module resolver in Node.js is 684allowed to be asynchronous.</p> 685</section><section><h3>Interoperability with CommonJS<span><a class="mark" href="#interoperability-with-commonjs" id="interoperability-with-commonjs">#</a></span><a aria-hidden="true" class="legacy" id="esm_interoperability_with_commonjs"></a></h3> 686<h4><code>import</code> statements<span><a class="mark" href="#import-statements" id="import-statements">#</a></span><a aria-hidden="true" class="legacy" id="esm_import_statements"></a></h4> 687<p>An <code>import</code> statement can reference an ES module or a CommonJS module. 688<code>import</code> statements are permitted only in ES modules, but dynamic <a href="#import-expressions"><code>import()</code></a> 689expressions are supported in CommonJS for loading ES modules.</p> 690<p>When importing <a href="#commonjs-namespaces">CommonJS modules</a>, the 691<code>module.exports</code> object is provided as the default export. Named exports may be 692available, provided by static analysis as a convenience for better ecosystem 693compatibility.</p> 694<h4><code>require</code><span><a class="mark" href="#require" id="require">#</a></span><a aria-hidden="true" class="legacy" id="esm_require"></a></h4> 695<p>The CommonJS module <code>require</code> always treats the files it references as CommonJS.</p> 696<p>Using <code>require</code> to load an ES module is not supported because ES modules have 697asynchronous execution. Instead, use <a href="#import-expressions"><code>import()</code></a> to load an ES module 698from a CommonJS module.</p> 699<h4>CommonJS Namespaces<span><a class="mark" href="#commonjs-namespaces" id="commonjs-namespaces">#</a></span><a aria-hidden="true" class="legacy" id="esm_commonjs_namespaces"></a></h4> 700<p>CommonJS modules consist of a <code>module.exports</code> object which can be of any type.</p> 701<p>When importing a CommonJS module, it can be reliably imported using the ES 702module default import or its corresponding sugar syntax:</p> 703<!-- eslint-disable no-duplicate-imports --> 704<pre><code class="language-js"><span class="hljs-keyword">import</span> { <span class="hljs-keyword">default</span> <span class="hljs-keyword">as</span> cjs } <span class="hljs-keyword">from</span> <span class="hljs-string">'cjs'</span>; 705 706<span class="hljs-comment">// The following import statement is "syntax sugar" (equivalent but sweeter)</span> 707<span class="hljs-comment">// for `{ default as cjsSugar }` in the above import statement:</span> 708<span class="hljs-keyword">import</span> cjsSugar <span class="hljs-keyword">from</span> <span class="hljs-string">'cjs'</span>; 709 710<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(cjs); 711<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(cjs === cjsSugar); 712<span class="hljs-comment">// Prints:</span> 713<span class="hljs-comment">// <module.exports></span> 714<span class="hljs-comment">// true</span></code> <button class="copy-button">copy</button></pre> 715<p>The ECMAScript Module Namespace representation of a CommonJS module is always 716a namespace with a <code>default</code> export key pointing to the CommonJS 717<code>module.exports</code> value.</p> 718<p>This Module Namespace Exotic Object can be directly observed either when using 719<code>import * as m from 'cjs'</code> or a dynamic import:</p> 720<!-- eslint-skip --> 721<pre><code class="language-js"><span class="hljs-keyword">import</span> * <span class="hljs-keyword">as</span> m <span class="hljs-keyword">from</span> <span class="hljs-string">'cjs'</span>; 722<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(m); 723<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(m === <span class="hljs-keyword">await</span> <span class="hljs-keyword">import</span>(<span class="hljs-string">'cjs'</span>)); 724<span class="hljs-comment">// Prints:</span> 725<span class="hljs-comment">// [Module] { default: <module.exports> }</span> 726<span class="hljs-comment">// true</span></code> <button class="copy-button">copy</button></pre> 727<p>For better compatibility with existing usage in the JS ecosystem, Node.js 728in addition attempts to determine the CommonJS named exports of every imported 729CommonJS module to provide them as separate ES module exports using a static 730analysis process.</p> 731<p>For example, consider a CommonJS module written:</p> 732<pre><code class="language-js cjs"><span class="hljs-comment">// cjs.cjs</span> 733<span class="hljs-built_in">exports</span>.<span class="hljs-property">name</span> = <span class="hljs-string">'exported'</span>;</code> <button class="copy-button">copy</button></pre> 734<p>The preceding module supports named imports in ES modules:</p> 735<!-- eslint-disable no-duplicate-imports --> 736<pre><code class="language-js"><span class="hljs-keyword">import</span> { name } <span class="hljs-keyword">from</span> <span class="hljs-string">'./cjs.cjs'</span>; 737<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(name); 738<span class="hljs-comment">// Prints: 'exported'</span> 739 740<span class="hljs-keyword">import</span> cjs <span class="hljs-keyword">from</span> <span class="hljs-string">'./cjs.cjs'</span>; 741<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(cjs); 742<span class="hljs-comment">// Prints: { name: 'exported' }</span> 743 744<span class="hljs-keyword">import</span> * <span class="hljs-keyword">as</span> m <span class="hljs-keyword">from</span> <span class="hljs-string">'./cjs.cjs'</span>; 745<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(m); 746<span class="hljs-comment">// Prints: [Module] { default: { name: 'exported' }, name: 'exported' }</span></code> <button class="copy-button">copy</button></pre> 747<p>As can be seen from the last example of the Module Namespace Exotic Object being 748logged, the <code>name</code> export is copied off of the <code>module.exports</code> object and set 749directly on the ES module namespace when the module is imported.</p> 750<p>Live binding updates or new exports added to <code>module.exports</code> are not detected 751for these named exports.</p> 752<p>The detection of named exports is based on common syntax patterns but does not 753always correctly detect named exports. In these cases, using the default 754import form described above can be a better option.</p> 755<p>Named exports detection covers many common export patterns, reexport patterns 756and build tool and transpiler outputs. See <a href="https://github.com/nodejs/cjs-module-lexer/tree/1.2.2">cjs-module-lexer</a> for the exact 757semantics implemented.</p> 758<h4>Differences between ES modules and CommonJS<span><a class="mark" href="#differences-between-es-modules-and-commonjs" id="differences-between-es-modules-and-commonjs">#</a></span><a aria-hidden="true" class="legacy" id="esm_differences_between_es_modules_and_commonjs"></a></h4> 759<h5>No <code>require</code>, <code>exports</code>, or <code>module.exports</code><span><a class="mark" href="#no-require-exports-or-moduleexports" id="no-require-exports-or-moduleexports">#</a></span><a aria-hidden="true" class="legacy" id="esm_no_require_exports_or_module_exports"></a></h5> 760<p>In most cases, the ES module <code>import</code> can be used to load CommonJS modules.</p> 761<p>If needed, a <code>require</code> function can be constructed within an ES module using 762<a href="module.html#modulecreaterequirefilename"><code>module.createRequire()</code></a>.</p> 763<h5>No <code>__filename</code> or <code>__dirname</code><span><a class="mark" href="#no-__filename-or-__dirname" id="no-__filename-or-__dirname">#</a></span><a aria-hidden="true" class="legacy" id="esm_no_filename_or_dirname"></a></h5> 764<p>These CommonJS variables are not available in ES modules.</p> 765<p><code>__filename</code> and <code>__dirname</code> use cases can be replicated via 766<a href="#importmetaurl"><code>import.meta.url</code></a>.</p> 767<h5>No Addon Loading<span><a class="mark" href="#no-addon-loading" id="no-addon-loading">#</a></span><a aria-hidden="true" class="legacy" id="esm_no_addon_loading"></a></h5> 768<p><a href="addons.html">Addons</a> are not currently supported with ES module imports.</p> 769<p>They can instead be loaded with <a href="module.html#modulecreaterequirefilename"><code>module.createRequire()</code></a> or 770<a href="process.html#processdlopenmodule-filename-flags"><code>process.dlopen</code></a>.</p> 771<h5>No <code>require.resolve</code><span><a class="mark" href="#no-requireresolve" id="no-requireresolve">#</a></span><a aria-hidden="true" class="legacy" id="esm_no_require_resolve"></a></h5> 772<p>Relative resolution can be handled via <code>new URL('./local', import.meta.url)</code>.</p> 773<p>For a complete <code>require.resolve</code> replacement, there is a flagged experimental 774<a href="#importmetaresolvespecifier-parent"><code>import.meta.resolve</code></a> API.</p> 775<p>Alternatively <code>module.createRequire()</code> can be used.</p> 776<h5>No <code>NODE_PATH</code><span><a class="mark" href="#no-node_path" id="no-node_path">#</a></span><a aria-hidden="true" class="legacy" id="esm_no_node_path"></a></h5> 777<p><code>NODE_PATH</code> is not part of resolving <code>import</code> specifiers. Please use symlinks 778if this behavior is desired.</p> 779<h5>No <code>require.extensions</code><span><a class="mark" href="#no-requireextensions" id="no-requireextensions">#</a></span><a aria-hidden="true" class="legacy" id="esm_no_require_extensions"></a></h5> 780<p><code>require.extensions</code> is not used by <code>import</code>. The expectation is that loader 781hooks can provide this workflow in the future.</p> 782<h5>No <code>require.cache</code><span><a class="mark" href="#no-requirecache" id="no-requirecache">#</a></span><a aria-hidden="true" class="legacy" id="esm_no_require_cache"></a></h5> 783<p><code>require.cache</code> is not used by <code>import</code> as the ES module loader has its own 784separate cache.</p> 785<p><i id="esm_experimental_json_modules"></i></p> 786</section><section><h3>JSON modules<span><a class="mark" href="#json-modules" id="json-modules">#</a></span><a aria-hidden="true" class="legacy" id="esm_json_modules"></a></h3> 787<p></p><div class="api_stability api_stability_1"><a href="documentation.html#stability-index">Stability: 1</a> - Experimental</div><p></p> 788<p>JSON files can be referenced by <code>import</code>:</p> 789<pre><code class="language-js"><span class="hljs-keyword">import</span> packageConfig <span class="hljs-keyword">from</span> <span class="hljs-string">'./package.json'</span> assert { <span class="hljs-attr">type</span>: <span class="hljs-string">'json'</span> };</code> <button class="copy-button">copy</button></pre> 790<p>The <code>assert { type: 'json' }</code> syntax is mandatory; see <a href="#import-assertions">Import Assertions</a>.</p> 791<p>The imported JSON only exposes a <code>default</code> export. There is no support for named 792exports. A cache entry is created in the CommonJS cache to avoid duplication. 793The same object is returned in CommonJS if the JSON module has already been 794imported from the same path.</p> 795<p><i id="esm_experimental_wasm_modules"></i></p> 796</section><section><h3>Wasm modules<span><a class="mark" href="#wasm-modules" id="wasm-modules">#</a></span><a aria-hidden="true" class="legacy" id="esm_wasm_modules"></a></h3> 797<p></p><div class="api_stability api_stability_1"><a href="documentation.html#stability-index">Stability: 1</a> - Experimental</div><p></p> 798<p>Importing WebAssembly modules is supported under the 799<code>--experimental-wasm-modules</code> flag, allowing any <code>.wasm</code> files to be 800imported as normal modules while also supporting their module imports.</p> 801<p>This integration is in line with the 802<a href="https://github.com/webassembly/esm-integration">ES Module Integration Proposal for WebAssembly</a>.</p> 803<p>For example, an <code>index.mjs</code> containing:</p> 804<pre><code class="language-js"><span class="hljs-keyword">import</span> * <span class="hljs-keyword">as</span> M <span class="hljs-keyword">from</span> <span class="hljs-string">'./module.wasm'</span>; 805<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(M);</code> <button class="copy-button">copy</button></pre> 806<p>executed under:</p> 807<pre><code class="language-bash">node --experimental-wasm-modules index.mjs</code> <button class="copy-button">copy</button></pre> 808<p>would provide the exports interface for the instantiation of <code>module.wasm</code>.</p> 809<p><i id="esm_experimental_top_level_await"></i></p> 810</section><section><h3>Top-level <code>await</code><span><a class="mark" href="#top-level-await" id="top-level-await">#</a></span><a aria-hidden="true" class="legacy" id="esm_top_level_await"></a></h3> 811<div class="api_metadata"> 812<span>Added in: v14.8.0</span> 813</div> 814<p>The <code>await</code> keyword may be used in the top level body of an ECMAScript module.</p> 815<p>Assuming an <code>a.mjs</code> with</p> 816<pre><code class="language-js"><span class="hljs-keyword">export</span> <span class="hljs-keyword">const</span> five = <span class="hljs-keyword">await</span> <span class="hljs-title class_">Promise</span>.<span class="hljs-title function_">resolve</span>(<span class="hljs-number">5</span>);</code> <button class="copy-button">copy</button></pre> 817<p>And a <code>b.mjs</code> with</p> 818<pre><code class="language-js"><span class="hljs-keyword">import</span> { five } <span class="hljs-keyword">from</span> <span class="hljs-string">'./a.mjs'</span>; 819 820<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(five); <span class="hljs-comment">// Logs `5`</span></code> <button class="copy-button">copy</button></pre> 821<pre><code class="language-bash">node b.mjs <span class="hljs-comment"># works</span></code> <button class="copy-button">copy</button></pre> 822<p>If a top level <code>await</code> expression never resolves, the <code>node</code> process will exit 823with a <code>13</code> <a href="process.html#exit-codes">status code</a>.</p> 824<pre><code class="language-js"><span class="hljs-keyword">import</span> { spawn } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:child_process'</span>; 825<span class="hljs-keyword">import</span> { execPath } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:process'</span>; 826 827<span class="hljs-title function_">spawn</span>(execPath, [ 828 <span class="hljs-string">'--input-type=module'</span>, 829 <span class="hljs-string">'--eval'</span>, 830 <span class="hljs-comment">// Never-resolving Promise:</span> 831 <span class="hljs-string">'await new Promise(() => {})'</span>, 832]).<span class="hljs-title function_">once</span>(<span class="hljs-string">'exit'</span>, <span class="hljs-function">(<span class="hljs-params">code</span>) =></span> { 833 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(code); <span class="hljs-comment">// Logs `13`</span> 834});</code> <button class="copy-button">copy</button></pre> 835</section><section><h3>HTTPS and HTTP imports<span><a class="mark" href="#https-and-http-imports" id="https-and-http-imports">#</a></span><a aria-hidden="true" class="legacy" id="esm_https_and_http_imports"></a></h3> 836<p></p><div class="api_stability api_stability_1"><a href="documentation.html#stability-index">Stability: 1</a> - Experimental</div><p></p> 837<p>Importing network based modules using <code>https:</code> and <code>http:</code> is supported under 838the <code>--experimental-network-imports</code> flag. This allows web browser-like imports 839to work in Node.js with a few differences due to application stability and 840security concerns that are different when running in a privileged environment 841instead of a browser sandbox.</p> 842<h4>Imports are limited to HTTP/1<span><a class="mark" href="#imports-are-limited-to-http1" id="imports-are-limited-to-http1">#</a></span><a aria-hidden="true" class="legacy" id="esm_imports_are_limited_to_http_1"></a></h4> 843<p>Automatic protocol negotiation for HTTP/2 and HTTP/3 is not yet supported.</p> 844<h4>HTTP is limited to loopback addresses<span><a class="mark" href="#http-is-limited-to-loopback-addresses" id="http-is-limited-to-loopback-addresses">#</a></span><a aria-hidden="true" class="legacy" id="esm_http_is_limited_to_loopback_addresses"></a></h4> 845<p><code>http:</code> is vulnerable to man-in-the-middle attacks and is not allowed to be 846used for addresses outside of the IPv4 address <code>127.0.0.0/8</code> (<code>127.0.0.1</code> to 847<code>127.255.255.255</code>) and the IPv6 address <code>::1</code>. Support for <code>http:</code> is intended 848to be used for local development.</p> 849<h4>Authentication is never sent to the destination server.<span><a class="mark" href="#authentication-is-never-sent-to-the-destination-server" id="authentication-is-never-sent-to-the-destination-server">#</a></span><a aria-hidden="true" class="legacy" id="esm_authentication_is_never_sent_to_the_destination_server"></a></h4> 850<p><code>Authorization</code>, <code>Cookie</code>, and <code>Proxy-Authorization</code> headers are not sent to the 851server. Avoid including user info in parts of imported URLs. A security model 852for safely using these on the server is being worked on.</p> 853<h4>CORS is never checked on the destination server<span><a class="mark" href="#cors-is-never-checked-on-the-destination-server" id="cors-is-never-checked-on-the-destination-server">#</a></span><a aria-hidden="true" class="legacy" id="esm_cors_is_never_checked_on_the_destination_server"></a></h4> 854<p>CORS is designed to allow a server to limit the consumers of an API to a 855specific set of hosts. This is not supported as it does not make sense for a 856server-based implementation.</p> 857<h4>Cannot load non-network dependencies<span><a class="mark" href="#cannot-load-non-network-dependencies" id="cannot-load-non-network-dependencies">#</a></span><a aria-hidden="true" class="legacy" id="esm_cannot_load_non_network_dependencies"></a></h4> 858<p>These modules cannot access other modules that are not over <code>http:</code> or <code>https:</code>. 859To still access local modules while avoiding the security concern, pass in 860references to the local dependencies:</p> 861<pre><code class="language-js mjs"><span class="hljs-comment">// file.mjs</span> 862<span class="hljs-keyword">import</span> worker_threads <span class="hljs-keyword">from</span> <span class="hljs-string">'node:worker_threads'</span>; 863<span class="hljs-keyword">import</span> { configure, resize } <span class="hljs-keyword">from</span> <span class="hljs-string">'https://example.com/imagelib.mjs'</span>; 864<span class="hljs-title function_">configure</span>({ worker_threads });</code> <button class="copy-button">copy</button></pre> 865<pre><code class="language-js mjs"><span class="hljs-comment">// https://example.com/imagelib.mjs</span> 866<span class="hljs-keyword">let</span> worker_threads; 867<span class="hljs-keyword">export</span> <span class="hljs-keyword">function</span> <span class="hljs-title function_">configure</span>(<span class="hljs-params">opts</span>) { 868 worker_threads = opts.<span class="hljs-property">worker_threads</span>; 869} 870<span class="hljs-keyword">export</span> <span class="hljs-keyword">function</span> <span class="hljs-title function_">resize</span>(<span class="hljs-params">img, size</span>) { 871 <span class="hljs-comment">// Perform resizing in worker_thread to avoid main thread blocking</span> 872}</code> <button class="copy-button">copy</button></pre> 873<h4>Network-based loading is not enabled by default<span><a class="mark" href="#network-based-loading-is-not-enabled-by-default" id="network-based-loading-is-not-enabled-by-default">#</a></span><a aria-hidden="true" class="legacy" id="esm_network_based_loading_is_not_enabled_by_default"></a></h4> 874<p>For now, the <code>--experimental-network-imports</code> flag is required to enable loading 875resources over <code>http:</code> or <code>https:</code>. In the future, a different mechanism will be 876used to enforce this. Opt-in is required to prevent transitive dependencies 877inadvertently using potentially mutable state that could affect reliability 878of Node.js applications.</p> 879<p><i id="esm_experimental_loaders"></i></p> 880</section><section><h3>Loaders<span><a class="mark" href="#loaders" id="loaders">#</a></span><a aria-hidden="true" class="legacy" id="esm_loaders"></a></h3> 881<div class="api_metadata"> 882<details class="changelog"><summary>History</summary> 883<table> 884<tbody><tr><th>Version</th><th>Changes</th></tr> 885<tr><td>v18.6.0</td> 886<td><p>Add support for chaining loaders.</p></td></tr> 887<tr><td>v16.12.0</td> 888<td><p>Removed <code>getFormat</code>, <code>getSource</code>, <code>transformSource</code>, and <code>globalPreload</code>; added <code>load</code> hook and <code>getGlobalPreload</code> hook.</p></td></tr> 889<tr><td>v8.8.0</td> 890<td><p><span>Added in: v8.8.0</span></p></td></tr> 891</tbody></table> 892</details> 893</div> 894<p></p><div class="api_stability api_stability_1"><a href="documentation.html#stability-index">Stability: 1</a> - Experimental</div><p></p> 895<blockquote> 896<p>This API is currently being redesigned and will still change.</p> 897</blockquote> 898 899<p>To customize the default module resolution, loader hooks can optionally be 900provided via a <code>--experimental-loader ./loader-name.mjs</code> argument to Node.js.</p> 901<p>When hooks are used they apply to the entry point and all <code>import</code> calls. They 902won't apply to <code>require</code> calls; those still follow <a href="modules.html">CommonJS</a> rules.</p> 903<p>Loaders follow the pattern of <code>--require</code>:</p> 904<pre><code class="language-console">node \ 905 --experimental-loader unpkg \ 906 --experimental-loader http-to-https \ 907 --experimental-loader cache-buster</code> <button class="copy-button">copy</button></pre> 908<p>These are called in the following sequence: <code>cache-buster</code> calls 909<code>http-to-https</code> which calls <code>unpkg</code>.</p> 910<h4>Hooks<span><a class="mark" href="#hooks" id="hooks">#</a></span><a aria-hidden="true" class="legacy" id="esm_hooks"></a></h4> 911<p>Hooks are part of a chain, even if that chain consists of only one custom 912(user-provided) hook and the default hook, which is always present. Hook 913functions nest: each one must always return a plain object, and chaining happens 914as a result of each function calling <code>next<hookName>()</code>, which is a reference 915to the subsequent loader’s hook.</p> 916<p>A hook that returns a value lacking a required property triggers an exception. 917A hook that returns without calling <code>next<hookName>()</code> <em>and</em> without returning 918<code>shortCircuit: true</code> also triggers an exception. These errors are to help 919prevent unintentional breaks in the chain.</p> 920<h5><code>resolve(specifier, context, nextResolve)</code><span><a class="mark" href="#resolvespecifier-context-nextresolve" id="resolvespecifier-context-nextresolve">#</a></span><a aria-hidden="true" class="legacy" id="esm_resolve_specifier_context_nextresolve"></a></h5> 921<div class="api_metadata"> 922<details class="changelog"><summary>History</summary> 923<table> 924<tbody><tr><th>Version</th><th>Changes</th></tr> 925<tr><td>v18.6.0</td> 926<td><p>Add support for chaining resolve hooks. Each hook must either call <code>nextResolve()</code> or include a <code>shortCircuit</code> property set to <code>true</code> in its return.</p></td></tr> 927<tr><td>v17.1.0, v16.14.0</td> 928<td><p>Add support for import assertions.</p></td></tr> 929</tbody></table> 930</details> 931</div> 932<blockquote> 933<p>The loaders API is being redesigned. This hook may disappear or its 934signature may change. Do not rely on the API described below.</p> 935</blockquote> 936<ul> 937<li><code>specifier</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a></li> 938<li><code>context</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a> 939<ul> 940<li><code>conditions</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string[]></a> Export conditions of the relevant <code>package.json</code></li> 941<li><code>importAssertions</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a></li> 942<li><code>parentURL</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Undefined_type" class="type"><undefined></a> The module importing this one, or undefined 943if this is the Node.js entry point</li> 944</ul> 945</li> 946<li><code>nextResolve</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> The subsequent <code>resolve</code> hook in the chain, or the 947Node.js default <code>resolve</code> hook after the last user-supplied <code>resolve</code> hook 948<ul> 949<li><code>specifier</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a></li> 950<li><code>context</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a></li> 951</ul> 952</li> 953<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a> 954<ul> 955<li><code>format</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Null_type" class="type"><null></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Undefined_type" class="type"><undefined></a> A hint to the load hook (it might be 956ignored) 957<code>'builtin' | 'commonjs' | 'json' | 'module' | 'wasm'</code></li> 958<li><code>shortCircuit</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Undefined_type" class="type"><undefined></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Boolean_type" class="type"><boolean></a> A signal that this hook intends to 959terminate the chain of <code>resolve</code> hooks. <strong>Default:</strong> <code>false</code></li> 960<li><code>url</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> The absolute URL to which this input resolves</li> 961</ul> 962</li> 963</ul> 964<p>The <code>resolve</code> hook chain is responsible for resolving file URL for a given 965module specifier and parent URL, and optionally its format (such as <code>'module'</code>) 966as a hint to the <code>load</code> hook. If a format is specified, the <code>load</code> hook is 967ultimately responsible for providing the final <code>format</code> value (and it is free to 968ignore the hint provided by <code>resolve</code>); if <code>resolve</code> provides a <code>format</code>, a 969custom <code>load</code> hook is required even if only to pass the value to the Node.js 970default <code>load</code> hook.</p> 971<p>The module specifier is the string in an <code>import</code> statement or 972<code>import()</code> expression.</p> 973<p>The parent URL is the URL of the module that imported this one, or <code>undefined</code> 974if this is the main entry point for the application.</p> 975<p>The <code>conditions</code> property in <code>context</code> is an array of conditions for 976<a href="packages.html#conditional-exports">package exports conditions</a> that apply to this resolution 977request. They can be used for looking up conditional mappings elsewhere or to 978modify the list when calling the default resolution logic.</p> 979<p>The current <a href="packages.html#conditional-exports">package exports conditions</a> are always in 980the <code>context.conditions</code> array passed into the hook. To guarantee <em>default 981Node.js module specifier resolution behavior</em> when calling <code>defaultResolve</code>, the 982<code>context.conditions</code> array passed to it <em>must</em> include <em>all</em> elements of the 983<code>context.conditions</code> array originally passed into the <code>resolve</code> hook.</p> 984<pre><code class="language-js"><span class="hljs-keyword">export</span> <span class="hljs-keyword">async</span> <span class="hljs-keyword">function</span> <span class="hljs-title function_">resolve</span>(<span class="hljs-params">specifier, context, nextResolve</span>) { 985 <span class="hljs-keyword">const</span> { parentURL = <span class="hljs-literal">null</span> } = context; 986 987 <span class="hljs-keyword">if</span> (<span class="hljs-title class_">Math</span>.<span class="hljs-title function_">random</span>() > <span class="hljs-number">0.5</span>) { <span class="hljs-comment">// Some condition.</span> 988 <span class="hljs-comment">// For some or all specifiers, do some custom logic for resolving.</span> 989 <span class="hljs-comment">// Always return an object of the form {url: <string>}.</span> 990 <span class="hljs-keyword">return</span> { 991 <span class="hljs-attr">shortCircuit</span>: <span class="hljs-literal">true</span>, 992 <span class="hljs-attr">url</span>: parentURL ? 993 <span class="hljs-keyword">new</span> <span class="hljs-title function_">URL</span>(specifier, parentURL).<span class="hljs-property">href</span> : 994 <span class="hljs-keyword">new</span> <span class="hljs-title function_">URL</span>(specifier).<span class="hljs-property">href</span>, 995 }; 996 } 997 998 <span class="hljs-keyword">if</span> (<span class="hljs-title class_">Math</span>.<span class="hljs-title function_">random</span>() < <span class="hljs-number">0.5</span>) { <span class="hljs-comment">// Another condition.</span> 999 <span class="hljs-comment">// When calling `defaultResolve`, the arguments can be modified. In this</span> 1000 <span class="hljs-comment">// case it's adding another value for matching conditional exports.</span> 1001 <span class="hljs-keyword">return</span> <span class="hljs-title function_">nextResolve</span>(specifier, { 1002 ...context, 1003 <span class="hljs-attr">conditions</span>: [...context.<span class="hljs-property">conditions</span>, <span class="hljs-string">'another-condition'</span>], 1004 }); 1005 } 1006 1007 <span class="hljs-comment">// Defer to the next hook in the chain, which would be the</span> 1008 <span class="hljs-comment">// Node.js default resolve if this is the last user-specified loader.</span> 1009 <span class="hljs-keyword">return</span> <span class="hljs-title function_">nextResolve</span>(specifier); 1010}</code> <button class="copy-button">copy</button></pre> 1011<h5><code>load(url, context, nextLoad)</code><span><a class="mark" href="#loadurl-context-nextload" id="loadurl-context-nextload">#</a></span><a aria-hidden="true" class="legacy" id="esm_load_url_context_nextload"></a></h5> 1012<div class="api_metadata"> 1013<details class="changelog"><summary>History</summary> 1014<table> 1015<tbody><tr><th>Version</th><th>Changes</th></tr> 1016<tr><td>v18.6.0</td> 1017<td><p>Add support for chaining load hooks. Each hook must either call <code>nextLoad()</code> or include a <code>shortCircuit</code> property set to <code>true</code> in its return.</p></td></tr> 1018</tbody></table> 1019</details> 1020</div> 1021<blockquote> 1022<p>The loaders API is being redesigned. This hook may disappear or its 1023signature may change. Do not rely on the API described below.</p> 1024</blockquote> 1025<blockquote> 1026<p>In a previous version of this API, this was split across 3 separate, now 1027deprecated, hooks (<code>getFormat</code>, <code>getSource</code>, and <code>transformSource</code>).</p> 1028</blockquote> 1029<ul> 1030<li><code>url</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> The URL returned by the <code>resolve</code> chain</li> 1031<li><code>context</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a> 1032<ul> 1033<li><code>conditions</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string[]></a> Export conditions of the relevant <code>package.json</code></li> 1034<li><code>format</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Null_type" class="type"><null></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Undefined_type" class="type"><undefined></a> The format optionally supplied by the 1035<code>resolve</code> hook chain</li> 1036<li><code>importAssertions</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a></li> 1037</ul> 1038</li> 1039<li><code>nextLoad</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> The subsequent <code>load</code> hook in the chain, or the 1040Node.js default <code>load</code> hook after the last user-supplied <code>load</code> hook 1041<ul> 1042<li><code>specifier</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a></li> 1043<li><code>context</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a></li> 1044</ul> 1045</li> 1046<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a> 1047<ul> 1048<li><code>format</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a></li> 1049<li><code>shortCircuit</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Undefined_type" class="type"><undefined></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Boolean_type" class="type"><boolean></a> A signal that this hook intends to 1050terminate the chain of <code>resolve</code> hooks. <strong>Default:</strong> <code>false</code></li> 1051<li><code>source</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/ArrayBuffer" class="type"><ArrayBuffer></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray" class="type"><TypedArray></a> The source for Node.js to evaluate</li> 1052</ul> 1053</li> 1054</ul> 1055<p>The <code>load</code> hook provides a way to define a custom method of determining how 1056a URL should be interpreted, retrieved, and parsed. It is also in charge of 1057validating the import assertion.</p> 1058<p>The final value of <code>format</code> must be one of the following:</p> 1059 1060 1061 1062 1063 1064 1065 1066 1067 1068 1069 1070 1071 1072 1073 1074 1075 1076 1077 1078 1079 1080 1081 1082 1083 1084 1085 1086 1087 1088 1089 1090 1091 1092 1093 1094<table><thead><tr><th><code>format</code></th><th>Description</th><th>Acceptable types for <code>source</code> returned by <code>load</code></th></tr></thead><tbody><tr><td><code>'builtin'</code></td><td>Load a Node.js builtin module</td><td>Not applicable</td></tr><tr><td><code>'commonjs'</code></td><td>Load a Node.js CommonJS module</td><td>Not applicable</td></tr><tr><td><code>'json'</code></td><td>Load a JSON file</td><td>{ <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String"><code>string</code></a>, <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/ArrayBuffer"><code>ArrayBuffer</code></a>, <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray"><code>TypedArray</code></a> }</td></tr><tr><td><code>'module'</code></td><td>Load an ES module</td><td>{ <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String"><code>string</code></a>, <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/ArrayBuffer"><code>ArrayBuffer</code></a>, <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray"><code>TypedArray</code></a> }</td></tr><tr><td><code>'wasm'</code></td><td>Load a WebAssembly module</td><td>{ <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/ArrayBuffer"><code>ArrayBuffer</code></a>, <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray"><code>TypedArray</code></a> }</td></tr></tbody></table> 1095<p>The value of <code>source</code> is ignored for type <code>'builtin'</code> because currently it is 1096not possible to replace the value of a Node.js builtin (core) module. The value 1097of <code>source</code> is ignored for type <code>'commonjs'</code> because the CommonJS module loader 1098does not provide a mechanism for the ES module loader to override the 1099<a href="#commonjs-namespaces">CommonJS module return value</a>. This limitation might be 1100overcome in the future.</p> 1101<blockquote> 1102<p><strong>Caveat</strong>: The ESM <code>load</code> hook and namespaced exports from CommonJS modules 1103are incompatible. Attempting to use them together will result in an empty 1104object from the import. This may be addressed in the future.</p> 1105</blockquote> 1106<blockquote> 1107<p>These types all correspond to classes defined in ECMAScript.</p> 1108</blockquote> 1109<ul> 1110<li>The specific <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/ArrayBuffer"><code>ArrayBuffer</code></a> object is a <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/SharedArrayBuffer"><code>SharedArrayBuffer</code></a>.</li> 1111<li>The specific <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray"><code>TypedArray</code></a> object is a <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Uint8Array"><code>Uint8Array</code></a>.</li> 1112</ul> 1113<p>If the source value of a text-based format (i.e., <code>'json'</code>, <code>'module'</code>) 1114is not a string, it is converted to a string using <a href="util.html#class-utiltextdecoder"><code>util.TextDecoder</code></a>.</p> 1115<p>The <code>load</code> hook provides a way to define a custom method for retrieving the 1116source code of an ES module specifier. This would allow a loader to potentially 1117avoid reading files from disk. It could also be used to map an unrecognized 1118format to a supported one, for example <code>yaml</code> to <code>module</code>.</p> 1119<pre><code class="language-js"><span class="hljs-keyword">export</span> <span class="hljs-keyword">async</span> <span class="hljs-keyword">function</span> <span class="hljs-title function_">load</span>(<span class="hljs-params">url, context, nextLoad</span>) { 1120 <span class="hljs-keyword">const</span> { format } = context; 1121 1122 <span class="hljs-keyword">if</span> (<span class="hljs-title class_">Math</span>.<span class="hljs-title function_">random</span>() > <span class="hljs-number">0.5</span>) { <span class="hljs-comment">// Some condition</span> 1123 <span class="hljs-comment">/* 1124 For some or all URLs, do some custom logic for retrieving the source. 1125 Always return an object of the form { 1126 format: <string>, 1127 source: <string|buffer>, 1128 }. 1129 */</span> 1130 <span class="hljs-keyword">return</span> { 1131 format, 1132 <span class="hljs-attr">shortCircuit</span>: <span class="hljs-literal">true</span>, 1133 <span class="hljs-attr">source</span>: <span class="hljs-string">'...'</span>, 1134 }; 1135 } 1136 1137 <span class="hljs-comment">// Defer to the next hook in the chain.</span> 1138 <span class="hljs-keyword">return</span> <span class="hljs-title function_">nextLoad</span>(url); 1139}</code> <button class="copy-button">copy</button></pre> 1140<p>In a more advanced scenario, this can also be used to transform an unsupported 1141source to a supported one (see <a href="#examples">Examples</a> below).</p> 1142<h5><code>globalPreload()</code><span><a class="mark" href="#globalpreload" id="globalpreload">#</a></span><a aria-hidden="true" class="legacy" id="esm_globalpreload"></a></h5> 1143<div class="api_metadata"> 1144<details class="changelog"><summary>History</summary> 1145<table> 1146<tbody><tr><th>Version</th><th>Changes</th></tr> 1147<tr><td>v18.6.0</td> 1148<td><p>Add support for chaining globalPreload hooks.</p></td></tr> 1149</tbody></table> 1150</details> 1151</div> 1152<blockquote> 1153<p>The loaders API is being redesigned. This hook may disappear or its 1154signature may change. Do not rely on the API described below.</p> 1155</blockquote> 1156<blockquote> 1157<p>In a previous version of this API, this hook was named 1158<code>getGlobalPreloadCode</code>.</p> 1159</blockquote> 1160<ul> 1161<li><code>context</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a> Information to assist the preload code 1162<ul> 1163<li><code>port</code> <a href="worker_threads.html#class-messageport" class="type"><MessagePort></a></li> 1164</ul> 1165</li> 1166<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> Code to run before application startup</li> 1167</ul> 1168<p>Sometimes it might be necessary to run some code inside of the same global 1169scope that the application runs in. This hook allows the return of a string 1170that is run as a sloppy-mode script on startup.</p> 1171<p>Similar to how CommonJS wrappers work, the code runs in an implicit function 1172scope. The only argument is a <code>require</code>-like function that can be used to load 1173builtins like "fs": <code>getBuiltin(request: string)</code>.</p> 1174<p>If the code needs more advanced <code>require</code> features, it has to construct 1175its own <code>require</code> using <code>module.createRequire()</code>.</p> 1176<pre><code class="language-js"><span class="hljs-keyword">export</span> <span class="hljs-keyword">function</span> <span class="hljs-title function_">globalPreload</span>(<span class="hljs-params">context</span>) { 1177 <span class="hljs-keyword">return</span> <span class="hljs-string">`\ 1178globalThis.someInjectedProperty = 42; 1179console.log('I just set some globals!'); 1180 1181const { createRequire } = getBuiltin('module'); 1182const { cwd } = getBuiltin('process'); 1183 1184const require = createRequire(cwd() + '/<preload>'); 1185// [...] 1186`</span>; 1187}</code> <button class="copy-button">copy</button></pre> 1188<p>In order to allow communication between the application and the loader, another 1189argument is provided to the preload code: <code>port</code>. This is available as a 1190parameter to the loader hook and inside of the source text returned by the hook. 1191Some care must be taken in order to properly call <a href="https://nodejs.org/dist/latest-v17.x/docs/api/worker_threads.html#portref"><code>port.ref()</code></a> and 1192<a href="https://nodejs.org/dist/latest-v17.x/docs/api/worker_threads.html#portunref"><code>port.unref()</code></a> to prevent a process from being in a state where it won't 1193close normally.</p> 1194<pre><code class="language-js"><span class="hljs-comment">/** 1195 * This example has the application context send a message to the loader 1196 * and sends the message back to the application context 1197 */</span> 1198<span class="hljs-keyword">export</span> <span class="hljs-keyword">function</span> <span class="hljs-title function_">globalPreload</span>(<span class="hljs-params">{ port }</span>) { 1199 port.<span class="hljs-property">onmessage</span> = <span class="hljs-function">(<span class="hljs-params">evt</span>) =></span> { 1200 port.<span class="hljs-title function_">postMessage</span>(evt.<span class="hljs-property">data</span>); 1201 }; 1202 <span class="hljs-keyword">return</span> <span class="hljs-string">`\ 1203 port.postMessage('console.log("I went to the Loader and back");'); 1204 port.onmessage = (evt) => { 1205 eval(evt.data); 1206 }; 1207 `</span>; 1208}</code> <button class="copy-button">copy</button></pre> 1209<h4>Examples<span><a class="mark" href="#examples" id="examples">#</a></span><a aria-hidden="true" class="legacy" id="esm_examples"></a></h4> 1210<p>The various loader hooks can be used together to accomplish wide-ranging 1211customizations of the Node.js code loading and evaluation behaviors.</p> 1212<h5>HTTPS loader<span><a class="mark" href="#https-loader" id="https-loader">#</a></span><a aria-hidden="true" class="legacy" id="esm_https_loader"></a></h5> 1213<p>In current Node.js, specifiers starting with <code>https://</code> are experimental (see 1214<a href="#https-and-http-imports">HTTPS and HTTP imports</a>).</p> 1215<p>The loader below registers hooks to enable rudimentary support for such 1216specifiers. While this may seem like a significant improvement to Node.js core 1217functionality, there are substantial downsides to actually using this loader: 1218performance is much slower than loading files from disk, there is no caching, 1219and there is no security.</p> 1220<pre><code class="language-js"><span class="hljs-comment">// https-loader.mjs</span> 1221<span class="hljs-keyword">import</span> { get } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:https'</span>; 1222 1223<span class="hljs-keyword">export</span> <span class="hljs-keyword">function</span> <span class="hljs-title function_">load</span>(<span class="hljs-params">url, context, nextLoad</span>) { 1224 <span class="hljs-comment">// For JavaScript to be loaded over the network, we need to fetch and</span> 1225 <span class="hljs-comment">// return it.</span> 1226 <span class="hljs-keyword">if</span> (url.<span class="hljs-title function_">startsWith</span>(<span class="hljs-string">'https://'</span>)) { 1227 <span class="hljs-keyword">return</span> <span class="hljs-keyword">new</span> <span class="hljs-title class_">Promise</span>(<span class="hljs-function">(<span class="hljs-params">resolve, reject</span>) =></span> { 1228 <span class="hljs-title function_">get</span>(url, <span class="hljs-function">(<span class="hljs-params">res</span>) =></span> { 1229 <span class="hljs-keyword">let</span> data = <span class="hljs-string">''</span>; 1230 res.<span class="hljs-title function_">setEncoding</span>(<span class="hljs-string">'utf8'</span>); 1231 res.<span class="hljs-title function_">on</span>(<span class="hljs-string">'data'</span>, <span class="hljs-function">(<span class="hljs-params">chunk</span>) =></span> data += chunk); 1232 res.<span class="hljs-title function_">on</span>(<span class="hljs-string">'end'</span>, <span class="hljs-function">() =></span> <span class="hljs-title function_">resolve</span>({ 1233 <span class="hljs-comment">// This example assumes all network-provided JavaScript is ES module</span> 1234 <span class="hljs-comment">// code.</span> 1235 <span class="hljs-attr">format</span>: <span class="hljs-string">'module'</span>, 1236 <span class="hljs-attr">shortCircuit</span>: <span class="hljs-literal">true</span>, 1237 <span class="hljs-attr">source</span>: data, 1238 })); 1239 }).<span class="hljs-title function_">on</span>(<span class="hljs-string">'error'</span>, <span class="hljs-function">(<span class="hljs-params">err</span>) =></span> <span class="hljs-title function_">reject</span>(err)); 1240 }); 1241 } 1242 1243 <span class="hljs-comment">// Let Node.js handle all other URLs.</span> 1244 <span class="hljs-keyword">return</span> <span class="hljs-title function_">nextLoad</span>(url); 1245}</code> <button class="copy-button">copy</button></pre> 1246<pre><code class="language-js"><span class="hljs-comment">// main.mjs</span> 1247<span class="hljs-keyword">import</span> { <span class="hljs-variable constant_">VERSION</span> } <span class="hljs-keyword">from</span> <span class="hljs-string">'https://coffeescript.org/browser-compiler-modern/coffeescript.js'</span>; 1248 1249<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-variable constant_">VERSION</span>);</code> <button class="copy-button">copy</button></pre> 1250<p>With the preceding loader, running 1251<code>node --experimental-loader ./https-loader.mjs ./main.mjs</code> 1252prints the current version of CoffeeScript per the module at the URL in 1253<code>main.mjs</code>.</p> 1254<h5>Transpiler loader<span><a class="mark" href="#transpiler-loader" id="transpiler-loader">#</a></span><a aria-hidden="true" class="legacy" id="esm_transpiler_loader"></a></h5> 1255<p>Sources that are in formats Node.js doesn't understand can be converted into 1256JavaScript using the <a href="#loadurl-context-nextload"><code>load</code> hook</a>.</p> 1257<p>This is less performant than transpiling source files before running 1258Node.js; a transpiler loader should only be used for development and testing 1259purposes.</p> 1260<pre><code class="language-js"><span class="hljs-comment">// coffeescript-loader.mjs</span> 1261<span class="hljs-keyword">import</span> { readFile } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:fs/promises'</span>; 1262<span class="hljs-keyword">import</span> { dirname, extname, resolve <span class="hljs-keyword">as</span> resolvePath } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:path'</span>; 1263<span class="hljs-keyword">import</span> { cwd } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:process'</span>; 1264<span class="hljs-keyword">import</span> { fileURLToPath, pathToFileURL } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:url'</span>; 1265<span class="hljs-keyword">import</span> <span class="hljs-title class_">CoffeeScript</span> <span class="hljs-keyword">from</span> <span class="hljs-string">'coffeescript'</span>; 1266 1267<span class="hljs-keyword">const</span> baseURL = <span class="hljs-title function_">pathToFileURL</span>(<span class="hljs-string">`<span class="hljs-subst">${cwd()}</span>/`</span>).<span class="hljs-property">href</span>; 1268 1269<span class="hljs-keyword">export</span> <span class="hljs-keyword">async</span> <span class="hljs-keyword">function</span> <span class="hljs-title function_">load</span>(<span class="hljs-params">url, context, nextLoad</span>) { 1270 <span class="hljs-keyword">if</span> (extensionsRegex.<span class="hljs-title function_">test</span>(url)) { 1271 <span class="hljs-comment">// Now that we patched resolve to let CoffeeScript URLs through, we need to</span> 1272 <span class="hljs-comment">// tell Node.js what format such URLs should be interpreted as. Because</span> 1273 <span class="hljs-comment">// CoffeeScript transpiles into JavaScript, it should be one of the two</span> 1274 <span class="hljs-comment">// JavaScript formats: 'commonjs' or 'module'.</span> 1275 1276 <span class="hljs-comment">// CoffeeScript files can be either CommonJS or ES modules, so we want any</span> 1277 <span class="hljs-comment">// CoffeeScript file to be treated by Node.js the same as a .js file at the</span> 1278 <span class="hljs-comment">// same location. To determine how Node.js would interpret an arbitrary .js</span> 1279 <span class="hljs-comment">// file, search up the file system for the nearest parent package.json file</span> 1280 <span class="hljs-comment">// and read its "type" field.</span> 1281 <span class="hljs-keyword">const</span> format = <span class="hljs-keyword">await</span> <span class="hljs-title function_">getPackageType</span>(url); 1282 <span class="hljs-comment">// When a hook returns a format of 'commonjs', `source` is ignored.</span> 1283 <span class="hljs-comment">// To handle CommonJS files, a handler needs to be registered with</span> 1284 <span class="hljs-comment">// `require.extensions` in order to process the files with the CommonJS</span> 1285 <span class="hljs-comment">// loader. Avoiding the need for a separate CommonJS handler is a future</span> 1286 <span class="hljs-comment">// enhancement planned for ES module loaders.</span> 1287 <span class="hljs-keyword">if</span> (format === <span class="hljs-string">'commonjs'</span>) { 1288 <span class="hljs-keyword">return</span> { 1289 format, 1290 <span class="hljs-attr">shortCircuit</span>: <span class="hljs-literal">true</span>, 1291 }; 1292 } 1293 1294 <span class="hljs-keyword">const</span> { <span class="hljs-attr">source</span>: rawSource } = <span class="hljs-keyword">await</span> <span class="hljs-title function_">nextLoad</span>(url, { ...context, format }); 1295 <span class="hljs-comment">// This hook converts CoffeeScript source code into JavaScript source code</span> 1296 <span class="hljs-comment">// for all imported CoffeeScript files.</span> 1297 <span class="hljs-keyword">const</span> transformedSource = <span class="hljs-title function_">coffeeCompile</span>(rawSource.<span class="hljs-title function_">toString</span>(), url); 1298 1299 <span class="hljs-keyword">return</span> { 1300 format, 1301 <span class="hljs-attr">shortCircuit</span>: <span class="hljs-literal">true</span>, 1302 <span class="hljs-attr">source</span>: transformedSource, 1303 }; 1304 } 1305 1306 <span class="hljs-comment">// Let Node.js handle all other URLs.</span> 1307 <span class="hljs-keyword">return</span> <span class="hljs-title function_">nextLoad</span>(url); 1308} 1309 1310<span class="hljs-keyword">async</span> <span class="hljs-keyword">function</span> <span class="hljs-title function_">getPackageType</span>(<span class="hljs-params">url</span>) { 1311 <span class="hljs-comment">// `url` is only a file path during the first iteration when passed the</span> 1312 <span class="hljs-comment">// resolved url from the load() hook</span> 1313 <span class="hljs-comment">// an actual file path from load() will contain a file extension as it's</span> 1314 <span class="hljs-comment">// required by the spec</span> 1315 <span class="hljs-comment">// this simple truthy check for whether `url` contains a file extension will</span> 1316 <span class="hljs-comment">// work for most projects but does not cover some edge-cases (such as</span> 1317 <span class="hljs-comment">// extensionless files or a url ending in a trailing space)</span> 1318 <span class="hljs-keyword">const</span> isFilePath = !!<span class="hljs-title function_">extname</span>(url); 1319 <span class="hljs-comment">// If it is a file path, get the directory it's in</span> 1320 <span class="hljs-keyword">const</span> dir = isFilePath ? 1321 <span class="hljs-title function_">dirname</span>(<span class="hljs-title function_">fileURLToPath</span>(url)) : 1322 url; 1323 <span class="hljs-comment">// Compose a file path to a package.json in the same directory,</span> 1324 <span class="hljs-comment">// which may or may not exist</span> 1325 <span class="hljs-keyword">const</span> packagePath = <span class="hljs-title function_">resolvePath</span>(dir, <span class="hljs-string">'package.json'</span>); 1326 <span class="hljs-comment">// Try to read the possibly nonexistent package.json</span> 1327 <span class="hljs-keyword">const</span> type = <span class="hljs-keyword">await</span> <span class="hljs-title function_">readFile</span>(packagePath, { <span class="hljs-attr">encoding</span>: <span class="hljs-string">'utf8'</span> }) 1328 .<span class="hljs-title function_">then</span>(<span class="hljs-function">(<span class="hljs-params">filestring</span>) =></span> <span class="hljs-title class_">JSON</span>.<span class="hljs-title function_">parse</span>(filestring).<span class="hljs-property">type</span>) 1329 .<span class="hljs-title function_">catch</span>(<span class="hljs-function">(<span class="hljs-params">err</span>) =></span> { 1330 <span class="hljs-keyword">if</span> (err?.<span class="hljs-property">code</span> !== <span class="hljs-string">'ENOENT'</span>) <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">error</span>(err); 1331 }); 1332 <span class="hljs-comment">// Ff package.json existed and contained a `type` field with a value, voila</span> 1333 <span class="hljs-keyword">if</span> (type) <span class="hljs-keyword">return</span> type; 1334 <span class="hljs-comment">// Otherwise, (if not at the root) continue checking the next directory up</span> 1335 <span class="hljs-comment">// If at the root, stop and return false</span> 1336 <span class="hljs-keyword">return</span> dir.<span class="hljs-property">length</span> > <span class="hljs-number">1</span> && <span class="hljs-title function_">getPackageType</span>(<span class="hljs-title function_">resolvePath</span>(dir, <span class="hljs-string">'..'</span>)); 1337}</code> <button class="copy-button">copy</button></pre> 1338<pre><code class="language-coffee"><span class="hljs-comment"># main.coffee</span> 1339<span class="hljs-keyword">import</span> { scream } <span class="hljs-keyword">from</span> <span class="hljs-string">'./scream.coffee'</span> 1340console.log scream <span class="hljs-string">'hello, world'</span> 1341 1342<span class="hljs-keyword">import</span> { version } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:process'</span> 1343console.log <span class="hljs-string">"Brought to you by Node.js version <span class="hljs-subst">#{version}</span>"</span></code> <button class="copy-button">copy</button></pre> 1344<pre><code class="language-coffee"><span class="hljs-comment"># scream.coffee</span> 1345<span class="hljs-keyword">export</span> scream = <span class="hljs-function"><span class="hljs-params">(str)</span> -></span> str.toUpperCase()</code> <button class="copy-button">copy</button></pre> 1346<p>With the preceding loader, running 1347<code>node --experimental-loader ./coffeescript-loader.mjs main.coffee</code> 1348causes <code>main.coffee</code> to be turned into JavaScript after its source code is 1349loaded from disk but before Node.js executes it; and so on for any <code>.coffee</code>, 1350<code>.litcoffee</code> or <code>.coffee.md</code> files referenced via <code>import</code> statements of any 1351loaded file.</p> 1352<h5>"import map" loader<span><a class="mark" href="#import-map-loader" id="import-map-loader">#</a></span><a aria-hidden="true" class="legacy" id="esm_import_map_loader"></a></h5> 1353<p>The previous two loaders defined <code>load</code> hooks. This is an example of a loader 1354that does its work via the <code>resolve</code> hook. This loader reads an 1355<code>import-map.json</code> file that specifies which specifiers to override to another 1356URL (this is a very simplistic implemenation of a small subset of the 1357"import maps" specification).</p> 1358<pre><code class="language-js"><span class="hljs-comment">// import-map-loader.js</span> 1359<span class="hljs-keyword">import</span> fs <span class="hljs-keyword">from</span> <span class="hljs-string">'node:fs/promises'</span>; 1360 1361<span class="hljs-keyword">const</span> { imports } = <span class="hljs-title class_">JSON</span>.<span class="hljs-title function_">parse</span>(<span class="hljs-keyword">await</span> fs.<span class="hljs-title function_">readFile</span>(<span class="hljs-string">'import-map.json'</span>)); 1362 1363<span class="hljs-keyword">export</span> <span class="hljs-keyword">async</span> <span class="hljs-keyword">function</span> <span class="hljs-title function_">resolve</span>(<span class="hljs-params">specifier, context, nextResolve</span>) { 1364 <span class="hljs-keyword">if</span> (<span class="hljs-title class_">Object</span>.<span class="hljs-title function_">hasOwn</span>(imports, specifier)) { 1365 <span class="hljs-keyword">return</span> <span class="hljs-title function_">nextResolve</span>(imports[specifier], context); 1366 } 1367 1368 <span class="hljs-keyword">return</span> <span class="hljs-title function_">nextResolve</span>(specifier, context); 1369}</code> <button class="copy-button">copy</button></pre> 1370<p>Let's assume we have these files:</p> 1371<pre><code class="language-js"><span class="hljs-comment">// main.js</span> 1372<span class="hljs-keyword">import</span> <span class="hljs-string">'a-module'</span>;</code> <button class="copy-button">copy</button></pre> 1373<pre><code class="language-json"><span class="hljs-comment">// import-map.json</span> 1374<span class="hljs-punctuation">{</span> 1375 <span class="hljs-attr">"imports"</span><span class="hljs-punctuation">:</span> <span class="hljs-punctuation">{</span> 1376 <span class="hljs-attr">"a-module"</span><span class="hljs-punctuation">:</span> <span class="hljs-string">"./some-module.js"</span> 1377 <span class="hljs-punctuation">}</span> 1378<span class="hljs-punctuation">}</span></code> <button class="copy-button">copy</button></pre> 1379<pre><code class="language-js"><span class="hljs-comment">// some-module.js</span> 1380<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'some module!'</span>);</code> <button class="copy-button">copy</button></pre> 1381<p>If you run <code>node --experimental-loader ./import-map-loader.js main.js</code> 1382the output will be <code>some module!</code>.</p> 1383</section><section><h3>Resolution and loading algorithm<span><a class="mark" href="#resolution-and-loading-algorithm" id="resolution-and-loading-algorithm">#</a></span><a aria-hidden="true" class="legacy" id="esm_resolution_and_loading_algorithm"></a></h3> 1384<h4>Features<span><a class="mark" href="#features" id="features">#</a></span><a aria-hidden="true" class="legacy" id="esm_features"></a></h4> 1385<p>The default resolver has the following properties:</p> 1386<ul> 1387<li>FileURL-based resolution as is used by ES modules</li> 1388<li>Relative and absolute URL resolution</li> 1389<li>No default extensions</li> 1390<li>No folder mains</li> 1391<li>Bare specifier package resolution lookup through node_modules</li> 1392<li>Does not fail on unknown extensions or protocols</li> 1393<li>Can optionally provide a hint of the format to the loading phase</li> 1394</ul> 1395<p>The default loader has the following properties</p> 1396<ul> 1397<li>Support for builtin module loading via <code>node:</code> URLs</li> 1398<li>Support for "inline" module loading via <code>data:</code> URLs</li> 1399<li>Support for <code>file:</code> module loading</li> 1400<li>Fails on any other URL protocol</li> 1401<li>Fails on unknown extensions for <code>file:</code> loading 1402(supports only <code>.cjs</code>, <code>.js</code>, and <code>.mjs</code>)</li> 1403</ul> 1404<h4>Resolution algorithm<span><a class="mark" href="#resolution-algorithm" id="resolution-algorithm">#</a></span><a aria-hidden="true" class="legacy" id="esm_resolution_algorithm"></a></h4> 1405<p>The algorithm to load an ES module specifier is given through the 1406<strong>ESM_RESOLVE</strong> method below. It returns the resolved URL for a 1407module specifier relative to a parentURL.</p> 1408<p>The resolution algorithm determines the full resolved URL for a module 1409load, along with its suggested module format. The resolution algorithm 1410does not determine whether the resolved URL protocol can be loaded, 1411or whether the file extensions are permitted, instead these validations 1412are applied by Node.js during the load phase 1413(for example, if it was asked to load a URL that has a protocol that is 1414not <code>file:</code>, <code>data:</code>, <code>node:</code>, or if <code>--experimental-network-imports</code> 1415is enabled, <code>https:</code>).</p> 1416<p>The algorithm also tries to determine the format of the file based 1417on the extension (see <code>ESM_FILE_FORMAT</code> algorithm below). If it does 1418not recognize the file extension (eg if it is not <code>.mjs</code>, <code>.cjs</code>, or 1419<code>.json</code>), then a format of <code>undefined</code> is returned, 1420which will throw during the load phase.</p> 1421<p>The algorithm to determine the module format of a resolved URL is 1422provided by <strong>ESM_FILE_FORMAT</strong>, which returns the unique module 1423format for any file. The <em>"module"</em> format is returned for an ECMAScript 1424Module, while the <em>"commonjs"</em> format is used to indicate loading through the 1425legacy CommonJS loader. Additional formats such as <em>"addon"</em> can be extended in 1426future updates.</p> 1427<p>In the following algorithms, all subroutine errors are propagated as errors 1428of these top-level routines unless stated otherwise.</p> 1429<p><em>defaultConditions</em> is the conditional environment name array, 1430<code>["node", "import"]</code>.</p> 1431<p>The resolver can throw the following errors:</p> 1432<ul> 1433<li><em>Invalid Module Specifier</em>: Module specifier is an invalid URL, package name 1434or package subpath specifier.</li> 1435<li><em>Invalid Package Configuration</em>: package.json configuration is invalid or 1436contains an invalid configuration.</li> 1437<li><em>Invalid Package Target</em>: Package exports or imports define a target module 1438for the package that is an invalid type or string target.</li> 1439<li><em>Package Path Not Exported</em>: Package exports do not define or permit a target 1440subpath in the package for the given module.</li> 1441<li><em>Package Import Not Defined</em>: Package imports do not define the specifier.</li> 1442<li><em>Module Not Found</em>: The package or module requested does not exist.</li> 1443<li><em>Unsupported Directory Import</em>: The resolved path corresponds to a directory, 1444which is not a supported target for module imports.</li> 1445</ul> 1446<h4>Resolution Algorithm Specification<span><a class="mark" href="#resolution-algorithm-specification" id="resolution-algorithm-specification">#</a></span><a aria-hidden="true" class="legacy" id="esm_resolution_algorithm_specification"></a></h4> 1447<p><strong>ESM_RESOLVE</strong>(<em>specifier</em>, <em>parentURL</em>)</p> 1448<blockquote> 1449<ol> 1450<li>Let <em>resolved</em> be <strong>undefined</strong>.</li> 1451<li>If <em>specifier</em> is a valid URL, then 1452<ol> 1453<li>Set <em>resolved</em> to the result of parsing and reserializing 1454<em>specifier</em> as a URL.</li> 1455</ol> 1456</li> 1457<li>Otherwise, if <em>specifier</em> starts with <em>"/"</em>, <em>"./"</em>, or <em>"../"</em>, then 1458<ol> 1459<li>Set <em>resolved</em> to the URL resolution of <em>specifier</em> relative to 1460<em>parentURL</em>.</li> 1461</ol> 1462</li> 1463<li>Otherwise, if <em>specifier</em> starts with <em>"#"</em>, then 1464<ol> 1465<li>Set <em>resolved</em> to the result of 1466<strong>PACKAGE_IMPORTS_RESOLVE</strong>(<em>specifier</em>, 1467<em>parentURL</em>, <em>defaultConditions</em>).</li> 1468</ol> 1469</li> 1470<li>Otherwise, 1471<ol> 1472<li>Note: <em>specifier</em> is now a bare specifier.</li> 1473<li>Set <em>resolved</em> the result of 1474<strong>PACKAGE_RESOLVE</strong>(<em>specifier</em>, <em>parentURL</em>).</li> 1475</ol> 1476</li> 1477<li>Let <em>format</em> be <strong>undefined</strong>.</li> 1478<li>If <em>resolved</em> is a <em>"file:"</em> URL, then 1479<ol> 1480<li>If <em>resolved</em> contains any percent encodings of <em>"/"</em> or <em>"\"</em> (<em>"%2F"</em> 1481and <em>"%5C"</em> respectively), then 1482<ol> 1483<li>Throw an <em>Invalid Module Specifier</em> error.</li> 1484</ol> 1485</li> 1486<li>If the file at <em>resolved</em> is a directory, then 1487<ol> 1488<li>Throw an <em>Unsupported Directory Import</em> error.</li> 1489</ol> 1490</li> 1491<li>If the file at <em>resolved</em> does not exist, then 1492<ol> 1493<li>Throw a <em>Module Not Found</em> error.</li> 1494</ol> 1495</li> 1496<li>Set <em>resolved</em> to the real path of <em>resolved</em>, maintaining the 1497same URL querystring and fragment components.</li> 1498<li>Set <em>format</em> to the result of <strong>ESM_FILE_FORMAT</strong>(<em>resolved</em>).</li> 1499</ol> 1500</li> 1501<li>Otherwise, 1502<ol> 1503<li>Set <em>format</em> the module format of the content type associated with the 1504URL <em>resolved</em>.</li> 1505</ol> 1506</li> 1507<li>Return <em>format</em> and <em>resolved</em> to the loading phase</li> 1508</ol> 1509</blockquote> 1510<p><strong>PACKAGE_RESOLVE</strong>(<em>packageSpecifier</em>, <em>parentURL</em>)</p> 1511<blockquote> 1512<ol> 1513<li>Let <em>packageName</em> be <strong>undefined</strong>.</li> 1514<li>If <em>packageSpecifier</em> is an empty string, then 1515<ol> 1516<li>Throw an <em>Invalid Module Specifier</em> error.</li> 1517</ol> 1518</li> 1519<li>If <em>packageSpecifier</em> is a Node.js builtin module name, then 1520<ol> 1521<li>Return the string <em>"node:"</em> concatenated with <em>packageSpecifier</em>.</li> 1522</ol> 1523</li> 1524<li>If <em>packageSpecifier</em> does not start with <em>"@"</em>, then 1525<ol> 1526<li>Set <em>packageName</em> to the substring of <em>packageSpecifier</em> until the first 1527<em>"/"</em> separator or the end of the string.</li> 1528</ol> 1529</li> 1530<li>Otherwise, 1531<ol> 1532<li>If <em>packageSpecifier</em> does not contain a <em>"/"</em> separator, then 1533<ol> 1534<li>Throw an <em>Invalid Module Specifier</em> error.</li> 1535</ol> 1536</li> 1537<li>Set <em>packageName</em> to the substring of <em>packageSpecifier</em> 1538until the second <em>"/"</em> separator or the end of the string.</li> 1539</ol> 1540</li> 1541<li>If <em>packageName</em> starts with <em>"."</em> or contains <em>"\"</em> or <em>"%"</em>, then 1542<ol> 1543<li>Throw an <em>Invalid Module Specifier</em> error.</li> 1544</ol> 1545</li> 1546<li>Let <em>packageSubpath</em> be <em>"."</em> concatenated with the substring of 1547<em>packageSpecifier</em> from the position at the length of <em>packageName</em>.</li> 1548<li>If <em>packageSubpath</em> ends in <em>"/"</em>, then 1549<ol> 1550<li>Throw an <em>Invalid Module Specifier</em> error.</li> 1551</ol> 1552</li> 1553<li>Let <em>selfUrl</em> be the result of 1554<strong>PACKAGE_SELF_RESOLVE</strong>(<em>packageName</em>, <em>packageSubpath</em>, <em>parentURL</em>).</li> 1555<li>If <em>selfUrl</em> is not <strong>undefined</strong>, return <em>selfUrl</em>.</li> 1556<li>While <em>parentURL</em> is not the file system root, 1557<ol> 1558<li>Let <em>packageURL</em> be the URL resolution of <em>"node_modules/"</em> 1559concatenated with <em>packageSpecifier</em>, relative to <em>parentURL</em>.</li> 1560<li>Set <em>parentURL</em> to the parent folder URL of <em>parentURL</em>.</li> 1561<li>If the folder at <em>packageURL</em> does not exist, then 1562<ol> 1563<li>Continue the next loop iteration.</li> 1564</ol> 1565</li> 1566<li>Let <em>pjson</em> be the result of <strong>READ_PACKAGE_JSON</strong>(<em>packageURL</em>).</li> 1567<li>If <em>pjson</em> is not <strong>null</strong> and <em>pjson</em>.<em>exports</em> is not <strong>null</strong> or 1568<strong>undefined</strong>, then 1569<ol> 1570<li>Return the result of <strong>PACKAGE_EXPORTS_RESOLVE</strong>(<em>packageURL</em>, 1571<em>packageSubpath</em>, <em>pjson.exports</em>, <em>defaultConditions</em>).</li> 1572</ol> 1573</li> 1574<li>Otherwise, if <em>packageSubpath</em> is equal to <em>"."</em>, then 1575<ol> 1576<li>If <em>pjson.main</em> is a string, then 1577<ol> 1578<li>Return the URL resolution of <em>main</em> in <em>packageURL</em>.</li> 1579</ol> 1580</li> 1581</ol> 1582</li> 1583<li>Otherwise, 1584<ol> 1585<li>Return the URL resolution of <em>packageSubpath</em> in <em>packageURL</em>.</li> 1586</ol> 1587</li> 1588</ol> 1589</li> 1590<li>Throw a <em>Module Not Found</em> error.</li> 1591</ol> 1592</blockquote> 1593<p><strong>PACKAGE_SELF_RESOLVE</strong>(<em>packageName</em>, <em>packageSubpath</em>, <em>parentURL</em>)</p> 1594<blockquote> 1595<ol> 1596<li>Let <em>packageURL</em> be the result of <strong>LOOKUP_PACKAGE_SCOPE</strong>(<em>parentURL</em>).</li> 1597<li>If <em>packageURL</em> is <strong>null</strong>, then 1598<ol> 1599<li>Return <strong>undefined</strong>.</li> 1600</ol> 1601</li> 1602<li>Let <em>pjson</em> be the result of <strong>READ_PACKAGE_JSON</strong>(<em>packageURL</em>).</li> 1603<li>If <em>pjson</em> is <strong>null</strong> or if <em>pjson</em>.<em>exports</em> is <strong>null</strong> or 1604<strong>undefined</strong>, then 1605<ol> 1606<li>Return <strong>undefined</strong>.</li> 1607</ol> 1608</li> 1609<li>If <em>pjson.name</em> is equal to <em>packageName</em>, then 1610<ol> 1611<li>Return the result of <strong>PACKAGE_EXPORTS_RESOLVE</strong>(<em>packageURL</em>, 1612<em>packageSubpath</em>, <em>pjson.exports</em>, <em>defaultConditions</em>).</li> 1613</ol> 1614</li> 1615<li>Otherwise, return <strong>undefined</strong>.</li> 1616</ol> 1617</blockquote> 1618<p><strong>PACKAGE_EXPORTS_RESOLVE</strong>(<em>packageURL</em>, <em>subpath</em>, <em>exports</em>, <em>conditions</em>)</p> 1619<blockquote> 1620<ol> 1621<li>If <em>exports</em> is an Object with both a key starting with <em>"."</em> and a key not 1622starting with <em>"."</em>, throw an <em>Invalid Package Configuration</em> error.</li> 1623<li>If <em>subpath</em> is equal to <em>"."</em>, then 1624<ol> 1625<li>Let <em>mainExport</em> be <strong>undefined</strong>.</li> 1626<li>If <em>exports</em> is a String or Array, or an Object containing no keys 1627starting with <em>"."</em>, then 1628<ol> 1629<li>Set <em>mainExport</em> to <em>exports</em>.</li> 1630</ol> 1631</li> 1632<li>Otherwise if <em>exports</em> is an Object containing a <em>"."</em> property, then 1633<ol> 1634<li>Set <em>mainExport</em> to <em>exports</em>[<em>"."</em>].</li> 1635</ol> 1636</li> 1637<li>If <em>mainExport</em> is not <strong>undefined</strong>, then 1638<ol> 1639<li>Let <em>resolved</em> be the result of <strong>PACKAGE_TARGET_RESOLVE</strong>( 1640<em>packageURL</em>, <em>mainExport</em>, <strong>null</strong>, <strong>false</strong>, <em>conditions</em>).</li> 1641<li>If <em>resolved</em> is not <strong>null</strong> or <strong>undefined</strong>, return <em>resolved</em>.</li> 1642</ol> 1643</li> 1644</ol> 1645</li> 1646<li>Otherwise, if <em>exports</em> is an Object and all keys of <em>exports</em> start with 1647<em>"."</em>, then 1648<ol> 1649<li>Let <em>matchKey</em> be the string <em>"./"</em> concatenated with <em>subpath</em>.</li> 1650<li>Let <em>resolved</em> be the result of <strong>PACKAGE_IMPORTS_EXPORTS_RESOLVE</strong>( 1651<em>matchKey</em>, <em>exports</em>, <em>packageURL</em>, <strong>false</strong>, <em>conditions</em>).</li> 1652<li>If <em>resolved</em> is not <strong>null</strong> or <strong>undefined</strong>, return <em>resolved</em>.</li> 1653</ol> 1654</li> 1655<li>Throw a <em>Package Path Not Exported</em> error.</li> 1656</ol> 1657</blockquote> 1658<p><strong>PACKAGE_IMPORTS_RESOLVE</strong>(<em>specifier</em>, <em>parentURL</em>, <em>conditions</em>)</p> 1659<blockquote> 1660<ol> 1661<li>Assert: <em>specifier</em> begins with <em>"#"</em>.</li> 1662<li>If <em>specifier</em> is exactly equal to <em>"#"</em> or starts with <em>"#/"</em>, then 1663<ol> 1664<li>Throw an <em>Invalid Module Specifier</em> error.</li> 1665</ol> 1666</li> 1667<li>Let <em>packageURL</em> be the result of <strong>LOOKUP_PACKAGE_SCOPE</strong>(<em>parentURL</em>).</li> 1668<li>If <em>packageURL</em> is not <strong>null</strong>, then 1669<ol> 1670<li>Let <em>pjson</em> be the result of <strong>READ_PACKAGE_JSON</strong>(<em>packageURL</em>).</li> 1671<li>If <em>pjson.imports</em> is a non-null Object, then 1672<ol> 1673<li>Let <em>resolved</em> be the result of 1674<strong>PACKAGE_IMPORTS_EXPORTS_RESOLVE</strong>( 1675<em>specifier</em>, <em>pjson.imports</em>, <em>packageURL</em>, <strong>true</strong>, <em>conditions</em>).</li> 1676<li>If <em>resolved</em> is not <strong>null</strong> or <strong>undefined</strong>, return <em>resolved</em>.</li> 1677</ol> 1678</li> 1679</ol> 1680</li> 1681<li>Throw a <em>Package Import Not Defined</em> error.</li> 1682</ol> 1683</blockquote> 1684<p><strong>PACKAGE_IMPORTS_EXPORTS_RESOLVE</strong>(<em>matchKey</em>, <em>matchObj</em>, <em>packageURL</em>, 1685<em>isImports</em>, <em>conditions</em>)</p> 1686<blockquote> 1687<ol> 1688<li>If <em>matchKey</em> is a key of <em>matchObj</em> and does not contain <em>"*"</em>, then 1689<ol> 1690<li>Let <em>target</em> be the value of <em>matchObj</em>[<em>matchKey</em>].</li> 1691<li>Return the result of <strong>PACKAGE_TARGET_RESOLVE</strong>(<em>packageURL</em>, 1692<em>target</em>, <strong>null</strong>, <em>isImports</em>, <em>conditions</em>).</li> 1693</ol> 1694</li> 1695<li>Let <em>expansionKeys</em> be the list of keys of <em>matchObj</em> containing only a 1696single <em>"*"</em>, sorted by the sorting function <strong>PATTERN_KEY_COMPARE</strong> 1697which orders in descending order of specificity.</li> 1698<li>For each key <em>expansionKey</em> in <em>expansionKeys</em>, do 1699<ol> 1700<li>Let <em>patternBase</em> be the substring of <em>expansionKey</em> up to but excluding 1701the first <em>"*"</em> character.</li> 1702<li>If <em>matchKey</em> starts with but is not equal to <em>patternBase</em>, then 1703<ol> 1704<li>Let <em>patternTrailer</em> be the substring of <em>expansionKey</em> from the 1705index after the first <em>"*"</em> character.</li> 1706<li>If <em>patternTrailer</em> has zero length, or if <em>matchKey</em> ends with 1707<em>patternTrailer</em> and the length of <em>matchKey</em> is greater than or 1708equal to the length of <em>expansionKey</em>, then 1709<ol> 1710<li>Let <em>target</em> be the value of <em>matchObj</em>[<em>expansionKey</em>].</li> 1711<li>Let <em>patternMatch</em> be the substring of <em>matchKey</em> starting at the 1712index of the length of <em>patternBase</em> up to the length of 1713<em>matchKey</em> minus the length of <em>patternTrailer</em>.</li> 1714<li>Return the result of <strong>PACKAGE_TARGET_RESOLVE</strong>(<em>packageURL</em>, 1715<em>target</em>, <em>patternMatch</em>, <em>isImports</em>, <em>conditions</em>).</li> 1716</ol> 1717</li> 1718</ol> 1719</li> 1720</ol> 1721</li> 1722<li>Return <strong>null</strong>.</li> 1723</ol> 1724</blockquote> 1725<p><strong>PATTERN_KEY_COMPARE</strong>(<em>keyA</em>, <em>keyB</em>)</p> 1726<blockquote> 1727<ol> 1728<li>Assert: <em>keyA</em> ends with <em>"/"</em> or contains only a single <em>"*"</em>.</li> 1729<li>Assert: <em>keyB</em> ends with <em>"/"</em> or contains only a single <em>"*"</em>.</li> 1730<li>Let <em>baseLengthA</em> be the index of <em>"*"</em> in <em>keyA</em> plus one, if <em>keyA</em> 1731contains <em>"*"</em>, or the length of <em>keyA</em> otherwise.</li> 1732<li>Let <em>baseLengthB</em> be the index of <em>"*"</em> in <em>keyB</em> plus one, if <em>keyB</em> 1733contains <em>"*"</em>, or the length of <em>keyB</em> otherwise.</li> 1734<li>If <em>baseLengthA</em> is greater than <em>baseLengthB</em>, return -1.</li> 1735<li>If <em>baseLengthB</em> is greater than <em>baseLengthA</em>, return 1.</li> 1736<li>If <em>keyA</em> does not contain <em>"*"</em>, return 1.</li> 1737<li>If <em>keyB</em> does not contain <em>"*"</em>, return -1.</li> 1738<li>If the length of <em>keyA</em> is greater than the length of <em>keyB</em>, return -1.</li> 1739<li>If the length of <em>keyB</em> is greater than the length of <em>keyA</em>, return 1.</li> 1740<li>Return 0.</li> 1741</ol> 1742</blockquote> 1743<p><strong>PACKAGE_TARGET_RESOLVE</strong>(<em>packageURL</em>, <em>target</em>, <em>patternMatch</em>, 1744<em>isImports</em>, <em>conditions</em>)</p> 1745<blockquote> 1746<ol> 1747<li>If <em>target</em> is a String, then 1748<ol> 1749<li>If <em>target</em> does not start with <em>"./"</em>, then 1750<ol> 1751<li>If <em>isImports</em> is <strong>false</strong>, or if <em>target</em> starts with <em>"../"</em> or 1752<em>"/"</em>, or if <em>target</em> is a valid URL, then 1753<ol> 1754<li>Throw an <em>Invalid Package Target</em> error.</li> 1755</ol> 1756</li> 1757<li>If <em>patternMatch</em> is a String, then 1758<ol> 1759<li>Return <strong>PACKAGE_RESOLVE</strong>(<em>target</em> with every instance of <em>"*"</em> 1760replaced by <em>patternMatch</em>, <em>packageURL</em> + <em>"/"</em>).</li> 1761</ol> 1762</li> 1763<li>Return <strong>PACKAGE_RESOLVE</strong>(<em>target</em>, <em>packageURL</em> + <em>"/"</em>).</li> 1764</ol> 1765</li> 1766<li>If <em>target</em> split on <em>"/"</em> or <em>"\"</em> contains any <em>""</em>, <em>"."</em>, <em>".."</em>, 1767or <em>"node_modules"</em> segments after the first <em>"."</em> segment, case 1768insensitive and including percent encoded variants, throw an <em>Invalid 1769Package Target</em> error.</li> 1770<li>Let <em>resolvedTarget</em> be the URL resolution of the concatenation of 1771<em>packageURL</em> and <em>target</em>.</li> 1772<li>Assert: <em>resolvedTarget</em> is contained in <em>packageURL</em>.</li> 1773<li>If <em>patternMatch</em> is <strong>null</strong>, then 1774<ol> 1775<li>Return <em>resolvedTarget</em>.</li> 1776</ol> 1777</li> 1778<li>If <em>patternMatch</em> split on <em>"/"</em> or <em>"\"</em> contains any <em>""</em>, <em>"."</em>, 1779<em>".."</em>, or <em>"node_modules"</em> segments, case insensitive and including 1780percent encoded variants, throw an <em>Invalid Module Specifier</em> error.</li> 1781<li>Return the URL resolution of <em>resolvedTarget</em> with every instance of 1782<em>"*"</em> replaced with <em>patternMatch</em>.</li> 1783</ol> 1784</li> 1785<li>Otherwise, if <em>target</em> is a non-null Object, then 1786<ol> 1787<li>If <em>exports</em> contains any index property keys, as defined in ECMA-262 1788<a href="https://tc39.es/ecma262/#integer-index">6.1.7 Array Index</a>, throw an <em>Invalid Package Configuration</em> error.</li> 1789<li>For each property <em>p</em> of <em>target</em>, in object insertion order as, 1790<ol> 1791<li>If <em>p</em> equals <em>"default"</em> or <em>conditions</em> contains an entry for <em>p</em>, 1792then 1793<ol> 1794<li>Let <em>targetValue</em> be the value of the <em>p</em> property in <em>target</em>.</li> 1795<li>Let <em>resolved</em> be the result of <strong>PACKAGE_TARGET_RESOLVE</strong>( 1796<em>packageURL</em>, <em>targetValue</em>, <em>patternMatch</em>, <em>isImports</em>, 1797<em>conditions</em>).</li> 1798<li>If <em>resolved</em> is equal to <strong>undefined</strong>, continue the loop.</li> 1799<li>Return <em>resolved</em>.</li> 1800</ol> 1801</li> 1802</ol> 1803</li> 1804<li>Return <strong>undefined</strong>.</li> 1805</ol> 1806</li> 1807<li>Otherwise, if <em>target</em> is an Array, then 1808<ol> 1809<li>If _target.length is zero, return <strong>null</strong>.</li> 1810<li>For each item <em>targetValue</em> in <em>target</em>, do 1811<ol> 1812<li>Let <em>resolved</em> be the result of <strong>PACKAGE_TARGET_RESOLVE</strong>( 1813<em>packageURL</em>, <em>targetValue</em>, <em>patternMatch</em>, <em>isImports</em>, 1814<em>conditions</em>), continuing the loop on any <em>Invalid Package Target</em> 1815error.</li> 1816<li>If <em>resolved</em> is <strong>undefined</strong>, continue the loop.</li> 1817<li>Return <em>resolved</em>.</li> 1818</ol> 1819</li> 1820<li>Return or throw the last fallback resolution <strong>null</strong> return or error.</li> 1821</ol> 1822</li> 1823<li>Otherwise, if <em>target</em> is <em>null</em>, return <strong>null</strong>.</li> 1824<li>Otherwise throw an <em>Invalid Package Target</em> error.</li> 1825</ol> 1826</blockquote> 1827<p><strong>ESM_FILE_FORMAT</strong>(<em>url</em>)</p> 1828<blockquote> 1829<ol> 1830<li>Assert: <em>url</em> corresponds to an existing file.</li> 1831<li>If <em>url</em> ends in <em>".mjs"</em>, then 1832<ol> 1833<li>Return <em>"module"</em>.</li> 1834</ol> 1835</li> 1836<li>If <em>url</em> ends in <em>".cjs"</em>, then 1837<ol> 1838<li>Return <em>"commonjs"</em>.</li> 1839</ol> 1840</li> 1841<li>If <em>url</em> ends in <em>".json"</em>, then 1842<ol> 1843<li>Return <em>"json"</em>.</li> 1844</ol> 1845</li> 1846<li>Let <em>packageURL</em> be the result of <strong>LOOKUP_PACKAGE_SCOPE</strong>(<em>url</em>).</li> 1847<li>Let <em>pjson</em> be the result of <strong>READ_PACKAGE_JSON</strong>(<em>packageURL</em>).</li> 1848<li>If <em>pjson?.type</em> exists and is <em>"module"</em>, then 1849<ol> 1850<li>If <em>url</em> ends in <em>".js"</em>, then 1851<ol> 1852<li>Return <em>"module"</em>.</li> 1853</ol> 1854</li> 1855<li>Return <strong>undefined</strong>.</li> 1856</ol> 1857</li> 1858<li>Otherwise, 1859<ol> 1860<li>Return <strong>undefined</strong>.</li> 1861</ol> 1862</li> 1863</ol> 1864</blockquote> 1865<p><strong>LOOKUP_PACKAGE_SCOPE</strong>(<em>url</em>)</p> 1866<blockquote> 1867<ol> 1868<li>Let <em>scopeURL</em> be <em>url</em>.</li> 1869<li>While <em>scopeURL</em> is not the file system root, 1870<ol> 1871<li>Set <em>scopeURL</em> to the parent URL of <em>scopeURL</em>.</li> 1872<li>If <em>scopeURL</em> ends in a <em>"node_modules"</em> path segment, return <strong>null</strong>.</li> 1873<li>Let <em>pjsonURL</em> be the resolution of <em>"package.json"</em> within 1874<em>scopeURL</em>.</li> 1875<li>if the file at <em>pjsonURL</em> exists, then 1876<ol> 1877<li>Return <em>scopeURL</em>.</li> 1878</ol> 1879</li> 1880</ol> 1881</li> 1882<li>Return <strong>null</strong>.</li> 1883</ol> 1884</blockquote> 1885<p><strong>READ_PACKAGE_JSON</strong>(<em>packageURL</em>)</p> 1886<blockquote> 1887<ol> 1888<li>Let <em>pjsonURL</em> be the resolution of <em>"package.json"</em> within <em>packageURL</em>.</li> 1889<li>If the file at <em>pjsonURL</em> does not exist, then 1890<ol> 1891<li>Return <strong>null</strong>.</li> 1892</ol> 1893</li> 1894<li>If the file at <em>packageURL</em> does not parse as valid JSON, then 1895<ol> 1896<li>Throw an <em>Invalid Package Configuration</em> error.</li> 1897</ol> 1898</li> 1899<li>Return the parsed JSON source of the file at <em>pjsonURL</em>.</li> 1900</ol> 1901</blockquote> 1902<h4>Customizing ESM specifier resolution algorithm<span><a class="mark" href="#customizing-esm-specifier-resolution-algorithm" id="customizing-esm-specifier-resolution-algorithm">#</a></span><a aria-hidden="true" class="legacy" id="esm_customizing_esm_specifier_resolution_algorithm"></a></h4> 1903<p></p><div class="api_stability api_stability_1"><a href="documentation.html#stability-index">Stability: 1</a> - Experimental</div><p></p> 1904<blockquote> 1905<p>Do not rely on this flag. We plan to remove it once the 1906<a href="#loaders">Loaders API</a> has advanced to the point that equivalent functionality can 1907be achieved via custom loaders.</p> 1908</blockquote> 1909<p>The current specifier resolution does not support all default behavior of 1910the CommonJS loader. One of the behavior differences is automatic resolution 1911of file extensions and the ability to import directories that have an index 1912file.</p> 1913<p>The <code>--experimental-specifier-resolution=[mode]</code> flag can be used to customize 1914the extension resolution algorithm. The default mode is <code>explicit</code>, which 1915requires the full path to a module be provided to the loader. To enable the 1916automatic extension resolution and importing from directories that include an 1917index file use the <code>node</code> mode.</p> 1918<pre><code class="language-console"><span class="hljs-meta prompt_">$ </span><span class="language-bash">node index.mjs</span> 1919success! 1920<span class="hljs-meta prompt_">$ </span><span class="language-bash">node index <span class="hljs-comment"># Failure!</span></span> 1921Error: Cannot find module 1922<span class="hljs-meta prompt_">$ </span><span class="language-bash">node --experimental-specifier-resolution=node index</span> 1923success!</code> <button class="copy-button">copy</button></pre> 1924<!-- Note: The cjs-module-lexer link should be kept in-sync with the deps version --></section> 1925 <!-- API END --> 1926 </div> 1927 </div> 1928 </div> 1929</body> 1930</html> 1931