eslint/lib/rules/no-cond-assign.js

   1 /**
   2  * @fileoverview Rule to flag assignment in a conditional statement's test expression
   3  * @author Stephen Murray <spmurrayzzz>
   4  */
   5
   6 "use strict";
   7
   8 //------------------------------------------------------------------------------
   9 // Requirements
  10 //------------------------------------------------------------------------------
  11
  12 const astUtils = require("./utils/ast-utils");
  13
  14 //------------------------------------------------------------------------------
  15 // Helpers
  16 //------------------------------------------------------------------------------
  17
  18 const TEST_CONDITION_PARENT_TYPES = new Set(["IfStatement", "WhileStatement", "DoWhileStatement", "ForStatement", "ConditionalExpression"]);
  19
  20 const NODE_DESCRIPTIONS = {
  21     DoWhileStatement: "a 'do...while' statement",
  22     ForStatement: "a 'for' statement",
  23     IfStatement: "an 'if' statement",
  24     WhileStatement: "a 'while' statement"
  25 };
  26
  27 //------------------------------------------------------------------------------
  28 // Rule Definition
  29 //------------------------------------------------------------------------------
  30
  31 module.exports = {
  32     meta: {
  33         type: "problem",
  34
  35         docs: {
  36             description: "disallow assignment operators in conditional expressions",
  37             recommended: true,
  38             url: "https://eslint.org/docs/rules/no-cond-assign"
  39         },
  40
  41         schema: [
  42             {
  43                 enum: ["except-parens", "always"]
  44             }
  45         ],
  46
  47         messages: {
  48             unexpected: "Unexpected assignment within {{type}}.",
  49
  50             // must match JSHint's error message
  51             missing: "Expected a conditional expression and instead saw an assignment."
  52         }
  53     },
  54
  55     create(context) {
  56
  57         const prohibitAssign = (context.options[0] || "except-parens");
  58
  59         const sourceCode = context.getSourceCode();
  60
  61         /**
  62          * Check whether an AST node is the test expression for a conditional statement.
  63          * @param {!Object} node The node to test.
  64          * @returns {boolean} `true` if the node is the text expression for a conditional statement; otherwise, `false`.
  65          */
  66         function isConditionalTestExpression(node) {
  67             return node.parent &&
  68                 TEST_CONDITION_PARENT_TYPES.has(node.parent.type) &&
  69                 node === node.parent.test;
  70         }
  71
  72         /**
  73          * Given an AST node, perform a bottom-up search for the first ancestor that represents a conditional statement.
  74          * @param {!Object} node The node to use at the start of the search.
  75          * @returns {?Object} The closest ancestor node that represents a conditional statement.
  76          */
  77         function findConditionalAncestor(node) {
  78             let currentAncestor = node;
  79
  80             do {
  81                 if (isConditionalTestExpression(currentAncestor)) {
  82                     return currentAncestor.parent;
  83                 }
  84             } while ((currentAncestor = currentAncestor.parent) && !astUtils.isFunction(currentAncestor));
  85
  86             return null;
  87         }
  88
  89         /**
  90          * Check whether the code represented by an AST node is enclosed in two sets of parentheses.
  91          * @param {!Object} node The node to test.
  92          * @returns {boolean} `true` if the code is enclosed in two sets of parentheses; otherwise, `false`.
  93          */
  94         function isParenthesisedTwice(node) {
  95             const previousToken = sourceCode.getTokenBefore(node, 1),
  96                 nextToken = sourceCode.getTokenAfter(node, 1);
  97
  98             return astUtils.isParenthesised(sourceCode, node) &&
  99                 previousToken && astUtils.isOpeningParenToken(previousToken) && previousToken.range[1] <= node.range[0] &&
 100                 astUtils.isClosingParenToken(nextToken) && nextToken.range[0] >= node.range[1];
 101         }
 102
 103         /**
 104          * Check a conditional statement's test expression for top-level assignments that are not enclosed in parentheses.
 105          * @param {!Object} node The node for the conditional statement.
 106          * @returns {void}
 107          */
 108         function testForAssign(node) {
 109             if (node.test &&
 110                 (node.test.type === "AssignmentExpression") &&
 111                 (node.type === "ForStatement"
 112                     ? !astUtils.isParenthesised(sourceCode, node.test)
 113                     : !isParenthesisedTwice(node.test)
 114                 )
 115             ) {
 116
 117                 context.report({
 118                     node: node.test,
 119                     messageId: "missing"
 120                 });
 121             }
 122         }
 123
 124         /**
 125          * Check whether an assignment expression is descended from a conditional statement's test expression.
 126          * @param {!Object} node The node for the assignment expression.
 127          * @returns {void}
 128          */
 129         function testForConditionalAncestor(node) {
 130             const ancestor = findConditionalAncestor(node);
 131
 132             if (ancestor) {
 133                 context.report({
 134                     node,
 135                     messageId: "unexpected",
 136                     data: {
 137                         type: NODE_DESCRIPTIONS[ancestor.type] || ancestor.type
 138                     }
 139                 });
 140             }
 141         }
 142
 143         if (prohibitAssign === "always") {
 144             return {
 145                 AssignmentExpression: testForConditionalAncestor
 146             };
 147         }
 148
 149         return {
 150             DoWhileStatement: testForAssign,
 151             ForStatement: testForAssign,
 152             IfStatement: testForAssign,
 153             WhileStatement: testForAssign,
 154             ConditionalExpression: testForAssign
 155         };
 156
 157     }
 158 };