eslint/lib/rules/no-plusplus.js

   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  */
  18 function 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  */
  34 function 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
  48 module.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 };