node/tools/eslint/lib/rules/comma-dangle.js

/**
 * @fileoverview Rule to forbid or enforce dangling commas.
 * @author Ian Christian Myers
 */

"use strict";

//------------------------------------------------------------------------------
// Requirements
//------------------------------------------------------------------------------

const lodash = require("lodash");

/**
 * Checks whether or not a trailing comma is allowed in a given node.
 * `ArrayPattern` which has `RestElement` disallows it.
 *
 * @param {ASTNode} node - A node to check.
 * @param {ASTNode} lastItem - The node of the last element in the given node.
 * @returns {boolean} `true` if a trailing comma is allowed.
 */
function isTrailingCommaAllowed(node, lastItem) {
    return node.type !== "ArrayPattern" || lastItem.type !== "RestElement";
}

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

module.exports = {
    meta: {
        docs: {
            description: "require or disallow trailing commas",
            category: "Stylistic Issues",
            recommended: false
        },

        fixable: "code",

        schema: [
            {
                enum: ["always", "always-multiline", "only-multiline", "never"]
            }
        ]
    },

    create: function(context) {
        const mode = context.options[0];
        const UNEXPECTED_MESSAGE = "Unexpected trailing comma.";
        const MISSING_MESSAGE = "Missing trailing comma.";

        /**
         * Checks whether or not a given node is multiline.
         * This rule handles a given node as multiline when the closing parenthesis
         * and the last element are not on the same line.
         *
         * @param {ASTNode} node - A node to check.
         * @returns {boolean} `true` if the node is multiline.
         */
        function isMultiline(node) {
            const lastItem = lodash.last(node.properties || node.elements || node.specifiers);

            if (!lastItem) {
                return false;
            }

            const sourceCode = context.getSourceCode();
            let penultimateToken = sourceCode.getLastToken(lastItem),
                lastToken = sourceCode.getTokenAfter(penultimateToken);

            // parentheses are a pain
            while (lastToken.value === ")") {
                penultimateToken = lastToken;
                lastToken = sourceCode.getTokenAfter(lastToken);
            }

            if (lastToken.value === ",") {
                penultimateToken = lastToken;
                lastToken = sourceCode.getTokenAfter(lastToken);
            }

            return lastToken.loc.end.line !== penultimateToken.loc.end.line;
        }

        /**
         * Reports a trailing comma if it exists.
         *
         * @param {ASTNode} node - A node to check. Its type is one of
         *   ObjectExpression, ObjectPattern, ArrayExpression, ArrayPattern,
         *   ImportDeclaration, and ExportNamedDeclaration.
         * @returns {void}
         */
        function forbidTrailingComma(node) {
            const lastItem = lodash.last(node.properties || node.elements || node.specifiers);

            if (!lastItem || (node.type === "ImportDeclaration" && lastItem.type !== "ImportSpecifier")) {
                return;
            }

            const sourceCode = context.getSourceCode();
            let trailingToken;

            // last item can be surrounded by parentheses for object and array literals
            if (node.type === "ObjectExpression" || node.type === "ArrayExpression") {
                trailingToken = sourceCode.getTokenBefore(sourceCode.getLastToken(node));
            } else {
                trailingToken = sourceCode.getTokenAfter(lastItem);
            }

            if (trailingToken.value === ",") {
                context.report({
                    node: lastItem,
                    loc: trailingToken.loc.start,
                    message: UNEXPECTED_MESSAGE,
                    fix: function(fixer) {
                        return fixer.remove(trailingToken);
                    }
                });
            }
        }

        /**
         * Reports the last element of a given node if it does not have a trailing
         * comma.
         *
         * If a given node is `ArrayPattern` which has `RestElement`, the trailing
         * comma is disallowed, so report if it exists.
         *
         * @param {ASTNode} node - A node to check. Its type is one of
         *   ObjectExpression, ObjectPattern, ArrayExpression, ArrayPattern,
         *   ImportDeclaration, and ExportNamedDeclaration.
         * @returns {void}
         */
        function forceTrailingComma(node) {
            const lastItem = lodash.last(node.properties || node.elements || node.specifiers);

            if (!lastItem || (node.type === "ImportDeclaration" && lastItem.type !== "ImportSpecifier")) {
                return;
            }
            if (!isTrailingCommaAllowed(node, lastItem)) {
                forbidTrailingComma(node);
                return;
            }

            const sourceCode = context.getSourceCode();
            let penultimateToken = lastItem,
                trailingToken = sourceCode.getTokenAfter(lastItem);

            // Skip close parentheses.
            while (trailingToken.value === ")") {
                penultimateToken = trailingToken;
                trailingToken = sourceCode.getTokenAfter(trailingToken);
            }

            if (trailingToken.value !== ",") {
                context.report({
                    node: lastItem,
                    loc: lastItem.loc.end,
                    message: MISSING_MESSAGE,
                    fix: function(fixer) {
                        return fixer.insertTextAfter(penultimateToken, ",");
                    }
                });
            }
        }

        /**
         * If a given node is multiline, reports the last element of a given node
         * when it does not have a trailing comma.
         * Otherwise, reports a trailing comma if it exists.
         *
         * @param {ASTNode} node - A node to check. Its type is one of
         *   ObjectExpression, ObjectPattern, ArrayExpression, ArrayPattern,
         *   ImportDeclaration, and ExportNamedDeclaration.
         * @returns {void}
         */
        function forceTrailingCommaIfMultiline(node) {
            if (isMultiline(node)) {
                forceTrailingComma(node);
            } else {
                forbidTrailingComma(node);
            }
        }

        /**
         * Only if a given node is not multiline, reports the last element of a given node
         * when it does not have a trailing comma.
         * Otherwise, reports a trailing comma if it exists.
         *
         * @param {ASTNode} node - A node to check. Its type is one of
         *   ObjectExpression, ObjectPattern, ArrayExpression, ArrayPattern,
         *   ImportDeclaration, and ExportNamedDeclaration.
         * @returns {void}
         */
        function allowTrailingCommaIfMultiline(node) {
            if (!isMultiline(node)) {
                forbidTrailingComma(node);
            }
        }

        // Chooses a checking function.
        let checkForTrailingComma;

        if (mode === "always") {
            checkForTrailingComma = forceTrailingComma;
        } else if (mode === "always-multiline") {
            checkForTrailingComma = forceTrailingCommaIfMultiline;
        } else if (mode === "only-multiline") {
            checkForTrailingComma = allowTrailingCommaIfMultiline;
        } else {
            checkForTrailingComma = forbidTrailingComma;
        }

        return {
            ObjectExpression: checkForTrailingComma,
            ObjectPattern: checkForTrailingComma,
            ArrayExpression: checkForTrailingComma,
            ArrayPattern: checkForTrailingComma,
            ImportDeclaration: checkForTrailingComma,
            ExportNamedDeclaration: checkForTrailingComma
        };
    }
};