node/tools/eslint/lib/rules/no-sequences.js

/**
 * @fileoverview Rule to flag use of comma operator
 * @author Brandon Mills
 */

"use strict";

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

module.exports = {
    meta: {
        docs: {
            description: "disallow comma operators",
            category: "Best Practices",
            recommended: false
        },

        schema: []
    },

    create: function(context) {
        const sourceCode = context.getSourceCode();

        /**
         * Parts of the grammar that are required to have parens.
         */
        const parenthesized = {
            DoWhileStatement: "test",
            IfStatement: "test",
            SwitchStatement: "discriminant",
            WhileStatement: "test",
            WithStatement: "object",
            ArrowFunctionExpression: "body"

            // Omitting CallExpression - commas are parsed as argument separators
            // Omitting NewExpression - commas are parsed as argument separators
            // Omitting ForInStatement - parts aren't individually parenthesised
            // Omitting ForStatement - parts aren't individually parenthesised
        };

        /**
         * Determines whether a node is required by the grammar to be wrapped in
         * parens, e.g. the test of an if statement.
         * @param {ASTNode} node - The AST node
         * @returns {boolean} True if parens around node belong to parent node.
         */
        function requiresExtraParens(node) {
            return node.parent && parenthesized[node.parent.type] &&
                    node === node.parent[parenthesized[node.parent.type]];
        }

        /**
         * Check if a node is wrapped in parens.
         * @param {ASTNode} node - The AST node
         * @returns {boolean} True if the node has a paren on each side.
         */
        function isParenthesised(node) {
            const previousToken = sourceCode.getTokenBefore(node),
                nextToken = sourceCode.getTokenAfter(node);

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

        /**
         * Check if a node is wrapped in two levels of parens.
         * @param {ASTNode} node - The AST node
         * @returns {boolean} True if two parens surround the node on each side.
         */
        function isParenthesisedTwice(node) {
            const previousToken = sourceCode.getTokenBefore(node, 1),
                nextToken = sourceCode.getTokenAfter(node, 1);

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

        return {
            SequenceExpression: function(node) {

                // Always allow sequences in for statement update
                if (node.parent.type === "ForStatement" &&
                        (node === node.parent.init || node === node.parent.update)) {
                    return;
                }

                // Wrapping a sequence in extra parens indicates intent
                if (requiresExtraParens(node)) {
                    if (isParenthesisedTwice(node)) {
                        return;
                    }
                } else {
                    if (isParenthesised(node)) {
                        return;
                    }
                }

                const child = sourceCode.getTokenAfter(node.expressions[0]);

                context.report(node, child.loc.start, "Unexpected use of comma operator.");
            }
        };

    }
};