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

/**
 * @fileoverview Rule to forbid or enforce dangling commas.
 * @author Ian Christian Myers
 * @copyright 2015 Mathias Schreck
 * @copyright 2013 Ian Christian Myers
 */

"use strict";

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

module.exports = function (context) {
    var allowDangle = context.options[0];
    var forbidDangle = allowDangle !== "always-multiline" && allowDangle !== "always";
    var UNEXPECTED_MESSAGE = "Unexpected trailing comma.";
    var MISSING_MESSAGE = "Missing trailing comma.";

    /**
     * Checks the given node for trailing comma and reports violations.
     * @param {ASTNode} node The node of an ObjectExpression or ArrayExpression
     * @returns {void}
     */
    function checkForTrailingComma(node) {
        var items = node.properties || node.elements,
            length = items.length,
            lastTokenOnNewLine,
            lastItem,
            penultimateToken,
            hasDanglingComma;

        if (length) {
            lastItem = items[length - 1];
            if (lastItem) {
                penultimateToken = context.getLastToken(node, 1);
                hasDanglingComma = penultimateToken.value === ",";

                if (forbidDangle && hasDanglingComma) {
                    context.report(lastItem, penultimateToken.loc.start, UNEXPECTED_MESSAGE);
                } else if (allowDangle === "always-multiline") {
                    lastTokenOnNewLine = node.loc.end.line !== penultimateToken.loc.end.line;
                    if (hasDanglingComma && !lastTokenOnNewLine) {
                        context.report(lastItem, penultimateToken.loc.start, UNEXPECTED_MESSAGE);
                    } else if (!hasDanglingComma && lastTokenOnNewLine) {
                        context.report(lastItem, penultimateToken.loc.end, MISSING_MESSAGE);
                    }
                } else if (allowDangle === "always" && !hasDanglingComma) {
                    context.report(lastItem, lastItem.loc.end, MISSING_MESSAGE);
                }
            }
        }
    }

    return {
        "ObjectExpression": checkForTrailingComma,
        "ArrayExpression": checkForTrailingComma
    };
};

module.exports.schema = [
    {
        "enum": ["always", "always-multiline", "never"]
    }
];