# JavaScript
You can use a .js file in the ECMAScript compliant JavaScript language to define the service logic of an HML page. With dynamic typing, JavaScript can make your application more expressive with a flexible design. The following describes the JavaScript compilation and running.
## Syntax
The ES6 syntax is supported.
- Module declaration
Import functionality modules.
```js
import router from '@ohos.router';
```
- Code reference
Import JavaScript code.
```js
import utils from '../../common/utils.js';
```
## Objects
- Application objects
| Name | Type | Description |
| -------- | -------- | -------- |
| $def | Object | Object that is exposed in the app.js file and obtained by `this.$app.$def`.<br/>> **NOTE**<br/>> Application objects do not support data binding. Data update should be triggered on the UI. |
Example
```js
// app.js
export default {
onCreate() {
console.info('Application onCreate');
},
onDestroy() {
console.info('Application onDestroy');
},
globalData: {
appData: 'appData',
appVersion: '2.0',
},
globalMethod() {
console.info('This is a global method!');
this.globalData.appVersion = '3.0';
}
};
```
```js
// index.js
export default {
data: {
appData: 'localData',
appVersion:'1.0',
},
onInit() {
this.appData = this.$app.$def.globalData.appData;
this.appVersion = this.$app.$def.globalData.appVersion;
},
invokeGlobalMethod() {
this.$app.$def.globalMethod();
},
getAppVersion() {
this.appVersion = this.$app.$def.globalData.appVersion;
}
}
```
- Page objects
| Name | Type | Description |
| -------- | -------- | -------- |
| data | Object/Function | Data model of the page. If the attribute is of the function type, the return value must be of the object type. The attribute name cannot start with a dollar sign ($) or underscore (_). Do not use reserved words (for, if, show, and tid).<br/>Do not use this attribute and private or public at the same time. |
| $refs | Object | DOM elements or child component instances that have registered the ref attribute. For example code, see [Obtaining a DOM element](#obtaining-a-dom-element). |
| private | Object | Data model of the page. Private data attribute can be modified only on the current page. |
| public | Object | Data model of the page. Behaviors of public data attributes are the same as those of the data attribute. |
| props | Array/Object | Used for communication between components. This attribute can be transferred to components via <tag xxxx='value'>. A props name must be in lowercase and cannot start with a dollar sign ($) or underscore (_). Do not use reserved words (for, if, show, and tid). Currently, props does not support functions. For details, see [props](../reference/arkui-js/js-components-custom-props.md). |
| computed | Object | Used for pre-processing an object for reading and setting. The result is cached. The name cannot start with a dollar sign ($) or underscore (_). Do not use reserved words. For details, see [props](../reference/arkui-js/js-components-custom-props.md). |
## Methods
- Data methods
| Name | Parameter | Description |
| -------- | -------- | -------- |
| $set | key: string, value: any | Adds an attribute or modifies an existing attribute.<br/>Usage:<br/>this.$set('_key_',_value_): Add an attribute. |
| $delete | key: string | Deletes an attribute.<br/>Usage:<br/>this.$delete('key'): Delete an attribute. |
Example
```js
// index.js
export default {
data: {
keyMap: {
OS: 'OpenHarmony',
Version: '2.0',
},
},
getAppVersion() {
this.$set('keyMap.Version', '3.0');
console.info("keyMap.Version = " + this.keyMap.Version); // keyMap.Version = 3.0
this.$delete('keyMap');
console.info("keyMap.Version = " + this.keyMap); // log print: keyMap.Version = undefined
}
}
```
- Public methods
| Name | Parameter | Description |
| -------- | -------- | -------- |
| $element | id: string | Obtains the component with a specified ID. If no ID is specified, the root component is returned. For example code, see [Obtaining a DOM element](#obtaining-a-dom-element).<br/>Usage:<br/>```<div id='_xxx_'></div>```<br/>- this.$element('_xxx_'): Obtain the component whose ID is _xxx_.<br/>- this.$element(): Obtain the root component. |
| $rootElement | None | Obtains the root element.<br/>Usage: this.$rootElement().scrollTo({ duration: 500, position: 300 }), which scrolls the page by 300 px within 500 ms. |
| $root | N/A | Obtains the root ViewModel instance. For example code, see [Obtaining the ViewModel](#obtaining-the-viewmodel). |
| $parent | N/A | Obtains the parent ViewModel instance. For example code, see [Obtaining the ViewModel](#obtaining-the-viewmodel). |
| $child | id: string | Obtains the ViewModel instance of a custom child component with a specified ID. For example code, see [Obtaining the ViewModel](#obtaining-the-viewmodel).<br/>Usage:<br/>this.$child('xxx'): Obtain the ViewModel instance of a custom child component whose ID is _xxx_. |
- Event methods
| Name | Parameter | Description |
| -------- | -------- | -------- |
| $watch | data: string, callback: string \| Function | Listens for attribute changes. If the value of the data attribute changes, the bound event is triggered. For details, see [props](../reference/arkui-js/js-components-custom-props.md)<br/>Usage:<br/>this.$watch('key', callback) |
- Page methods
| Name | Parameter | Description |
| -------- | -------- | -------- |
| scrollTo<sup>6+</sup> | scrollPageParam: ScrollPageParam | Scrolls the page to the target position. You can specify the position using the ID selector or scrolling distance. |
**Table 1** ScrollPageParam<sup>6+</sup>
| Name | Type | Default Value | Description |
| -------- | -------- | -------- | -------- |
| position | number | - | Position to scroll to. |
| id | string | - | ID of the element to be scrolled to. |
| duration | number | 300 | Scrolling duration, in milliseconds. |
| timingFunction | string | ease | Animation curve for scrolling. Available options:<br/>[Animation Styles](../reference/arkui-js/js-components-common-animation.md) |
| complete | () => void | - | Callback to be invoked when the scrolling is complete. |
Example
```js
this.$rootElement().scrollTo({position: 0})
this.$rootElement().scrollTo({id: 'id', duration: 200, timingFunction: 'ease-in', complete: ()=>void})
```
## Obtaining a DOM Element
1. Use `$refs` to obtain a DOM element.
```html
<!-- index.hml -->
<div class="container">
<image-animator class="image-player" ref="animator" images="{{images}}" duration="1s" onclick="handleClick"></image-animator>
</div>
```
```js
// index.js
export default {
data: {
images: [
{ src: '/common/frame1.png' },
{ src: '/common/frame2.png' },
{ src: '/common/frame3.png' }
]
},
handleClick() {
const animator = this.$refs.animator; // Obtain the DOM element whose $refs attribute is animator.
const state = animator.getState();
if (state === 'paused') {
animator.resume();
} else if (state === 'stopped') {
animator.start();
} else {
animator.pause();
}
},
};
```
2. Call `$element` to obtain a DOM element.
```html
<!-- index.hml -->
<div class="container" style="width:500px;height: 700px; margin: 100px;">
<image-animator class="image-player" id="animator" images="{{images}}" duration="1s" onclick="handleClick"></image-animator>
</div>
```
```js
// index.js
export default {
data: {
images: [
{ src: '/common/frame1.png' },
{ src: '/common/frame2.png' },
{ src: '/common/frame3.png' }
]
},
handleClick() {
const animator = this.$element('animator'); // Obtain the DOM element whose ID is animator.
const state = animator.getState();
if (state === 'paused') {
animator.resume();
} else if (state === 'stopped') {
animator.start();
} else {
animator.pause();
}
},
};
```
## Obtaining the ViewModel
The following shows files of the root page:
```html
<!-- root.hml -->
<element name='parentComp' src='../../common/component/parent/parent.hml'></element>
<div class="container">
<div class="container">
<text>{{text}}</text>
<parentComp></parentComp>
</div>
</div>
```
```js
// root.js
export default {
data: {
text: 'I am a root!',
},
}
```
Customize the parent component.
```html
<!-- parent.hml -->
<element name='childComp' src='../child/child.hml'></element>
<div class="item" onclick="textClicked">
<text class="text-style" onclick="parentClicked">parent component click</text>
<text class="text-style" if="{{showValue}}">hello parent component!</text>
<childComp id = "selfDefineChild"></childComp>
</div>
```
```js
// parent.js
export default {
data: {
showValue: false,
text: 'I am parent component!',
},
parentClicked () {
this.showValue = !this.showValue;
console.info('parent component get parent text');
console.info(`${this.$parent().text}`);
console.info("The parent component gets the child function.");
console.info(`${this.$child('selfDefineChild').childClicked()}`);
},
}
```
Customize the child component.
```html
<!-- child.hml -->
<div class="item" onclick="textClicked">
<text class="text-style" onclick="childClicked">Child component clicked</text>
<text class="text-style" if="{{isShow}}">Hello child component</text>
</div>
```
```js
// child.js
export default {
data: {
isShow: false,
text: 'I am the child component!',
},
childClicked () {
this.isShow = !this.isShow;
console.info('child component get parent text');
console.info('${this.$parent().text}');
console.info('child component get root text');
console.info('${this.$root().text}');
},
}
```
