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1/**
2 * @fileoverview Rule to flag use of unary increment and decrement operators.
3 * @author Ian Christian Myers
4 * @author Brody McKee (github.com/mrmckeb)
5 */
6
7"use strict";
8
9//------------------------------------------------------------------------------
10// Helpers
11//------------------------------------------------------------------------------
12
13/**
14 * Determines whether the given node is the update node of a `ForStatement`.
15 * @param {ASTNode} node The node to check.
16 * @returns {boolean} `true` if the node is `ForStatement` update.
17 */
18function isForStatementUpdate(node) {
19    const parent = node.parent;
20
21    return parent.type === "ForStatement" && parent.update === node;
22}
23
24/**
25 * Determines whether the given node is considered to be a for loop "afterthought" by the logic of this rule.
26 * In particular, it returns `true` if the given node is either:
27 *   - The update node of a `ForStatement`: for (;; i++) {}
28 *   - An operand of a sequence expression that is the update node: for (;; foo(), i++) {}
29 *   - An operand of a sequence expression that is child of another sequence expression, etc.,
30 *     up to the sequence expression that is the update node: for (;; foo(), (bar(), (baz(), i++))) {}
31 * @param {ASTNode} node The node to check.
32 * @returns {boolean} `true` if the node is a for loop afterthought.
33 */
34function isForLoopAfterthought(node) {
35    const parent = node.parent;
36
37    if (parent.type === "SequenceExpression") {
38        return isForLoopAfterthought(parent);
39    }
40
41    return isForStatementUpdate(node);
42}
43
44//------------------------------------------------------------------------------
45// Rule Definition
46//------------------------------------------------------------------------------
47
48module.exports = {
49    meta: {
50        type: "suggestion",
51
52        docs: {
53            description: "disallow the unary operators `++` and `--`",
54            category: "Stylistic Issues",
55            recommended: false,
56            url: "https://eslint.org/docs/rules/no-plusplus"
57        },
58
59        schema: [
60            {
61                type: "object",
62                properties: {
63                    allowForLoopAfterthoughts: {
64                        type: "boolean",
65                        default: false
66                    }
67                },
68                additionalProperties: false
69            }
70        ],
71
72        messages: {
73            unexpectedUnaryOp: "Unary operator '{{operator}}' used."
74        }
75    },
76
77    create(context) {
78
79        const config = context.options[0];
80        let allowForLoopAfterthoughts = false;
81
82        if (typeof config === "object") {
83            allowForLoopAfterthoughts = config.allowForLoopAfterthoughts === true;
84        }
85
86        return {
87
88            UpdateExpression(node) {
89                if (allowForLoopAfterthoughts && isForLoopAfterthought(node)) {
90                    return;
91                }
92
93                context.report({
94                    node,
95                    messageId: "unexpectedUnaryOp",
96                    data: {
97                        operator: node.operator
98                    }
99                });
100            }
101
102        };
103
104    }
105};
106