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

/**
 * @fileoverview Rule to flag assignment in a conditional statement's test expression
 * @author Stephen Murray <spmurrayzzz>
 */
"use strict";

const astUtils = require("../ast-utils");

const NODE_DESCRIPTIONS = {
    DoWhileStatement: "a 'do...while' statement",
    ForStatement: "a 'for' statement",
    IfStatement: "an 'if' statement",
    WhileStatement: "a 'while' statement"
};

//------------------------------------------------------------------------------
// Rule Definition
//------------------------------------------------------------------------------

module.exports = {
    meta: {
        docs: {
            description: "disallow assignment operators in conditional expressions",
            category: "Possible Errors",
            recommended: true
        },

        schema: [
            {
                enum: ["except-parens", "always"]
            }
        ]
    },

    create(context) {

        const prohibitAssign = (context.options[0] || "except-parens");

        const sourceCode = context.getSourceCode();

        /**
         * Check whether an AST node is the test expression for a conditional statement.
         * @param {!Object} node The node to test.
         * @returns {boolean} `true` if the node is the text expression for a conditional statement; otherwise, `false`.
         */
        function isConditionalTestExpression(node) {
            return node.parent &&
                node.parent.test &&
                node === node.parent.test;
        }

        /**
         * Given an AST node, perform a bottom-up search for the first ancestor that represents a conditional statement.
         * @param {!Object} node The node to use at the start of the search.
         * @returns {?Object} The closest ancestor node that represents a conditional statement.
         */
        function findConditionalAncestor(node) {
            let currentAncestor = node;

            do {
                if (isConditionalTestExpression(currentAncestor)) {
                    return currentAncestor.parent;
                }
            } while ((currentAncestor = currentAncestor.parent) && !astUtils.isFunction(currentAncestor));

            return null;
        }

        /**
         * Check whether the code represented by an AST node is enclosed in parentheses.
         * @param {!Object} node The node to test.
         * @returns {boolean} `true` if the code is enclosed in parentheses; otherwise, `false`.
         */
        function isParenthesised(node) {
            const previousToken = sourceCode.getTokenBefore(node),
                nextToken = sourceCode.getTokenAfter(node);

            return previousToken.value === "(" && previousToken.range[1] <= node.range[0] &&
                nextToken.value === ")" && nextToken.range[0] >= node.range[1];
        }

        /**
         * Check whether the code represented by an AST node is enclosed in two sets of parentheses.
         * @param {!Object} node The node to test.
         * @returns {boolean} `true` if the code is enclosed in two sets of parentheses; otherwise, `false`.
         */
        function isParenthesisedTwice(node) {
            const previousToken = sourceCode.getTokenBefore(node, 1),
                nextToken = sourceCode.getTokenAfter(node, 1);

            return isParenthesised(node) &&
                previousToken.value === "(" && previousToken.range[1] <= node.range[0] &&
                nextToken.value === ")" && nextToken.range[0] >= node.range[1];
        }

        /**
         * Check a conditional statement's test expression for top-level assignments that are not enclosed in parentheses.
         * @param {!Object} node The node for the conditional statement.
         * @returns {void}
         */
        function testForAssign(node) {
            if (node.test &&
                (node.test.type === "AssignmentExpression") &&
                (node.type === "ForStatement" ?
                    !isParenthesised(node.test) :
                    !isParenthesisedTwice(node.test)
                )
            ) {

                // must match JSHint's error message
                context.report({
                    node,
                    loc: node.test.loc.start,
                    message: "Expected a conditional expression and instead saw an assignment."
                });
            }
        }

        /**
         * Check whether an assignment expression is descended from a conditional statement's test expression.
         * @param {!Object} node The node for the assignment expression.
         * @returns {void}
         */
        function testForConditionalAncestor(node) {
            const ancestor = findConditionalAncestor(node);

            if (ancestor) {
                context.report(ancestor, "Unexpected assignment within {{type}}.", {
                    type: NODE_DESCRIPTIONS[ancestor.type] || ancestor.type
                });
            }
        }

        if (prohibitAssign === "always") {
            return {
                AssignmentExpression: testForConditionalAncestor
            };
        }

        return {
            DoWhileStatement: testForAssign,
            ForStatement: testForAssign,
            IfStatement: testForAssign,
            WhileStatement: testForAssign
        };

    }
};