eslint/lib/rules/no-underscore-dangle.js

/**
 * @fileoverview Rule to flag trailing underscores in variable declarations.
 * @author Matt DuVall <http://www.mattduvall.com>
 */

"use strict";

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

module.exports = function(context) {

    var options = context.options[0] || {};
    var ALLOWED_VARIABLES = options.allow ? options.allow : [];
    var allowAfterThis = typeof options.allowAfterThis !== "undefined" ? options.allowAfterThis : false;

    //-------------------------------------------------------------------------
    // Helpers
    //-------------------------------------------------------------------------

    /**
     * Check if identifier is present inside the allowed option
     * @param {string} identifier name of the node
     * @returns {boolean} true if its is present
     * @private
     */
    function isAllowed(identifier) {
        return ALLOWED_VARIABLES.some(function(ident) {
            return ident === identifier;
        });
    }

    /**
     * Check if identifier has a underscore at the end
     * @param {ASTNode} identifier node to evaluate
     * @returns {boolean} true if its is present
     * @private
     */
    function hasTrailingUnderscore(identifier) {
        var len = identifier.length;

        return identifier !== "_" && (identifier[0] === "_" || identifier[len - 1] === "_");
    }

    /**
     * Check if identifier is a special case member expression
     * @param {ASTNode} identifier node to evaluate
     * @returns {boolean} true if its is a special case
     * @private
     */
    function isSpecialCaseIdentifierForMemberExpression(identifier) {
        return identifier === "__proto__";
    }

    /**
     * Check if identifier is a special case variable expression
     * @param {ASTNode} identifier node to evaluate
     * @returns {boolean} true if its is a special case
     * @private
     */
    function isSpecialCaseIdentifierInVariableExpression(identifier) {
        // Checks for the underscore library usage here
        return identifier === "_";
    }

    /**
     * Check if function has a underscore at the end
     * @param {ASTNode} node node to evaluate
     * @returns {void}
     * @private
     */
    function checkForTrailingUnderscoreInFunctionDeclaration(node) {
        if (node.id) {
            var identifier = node.id.name;

            if (typeof identifier !== "undefined" && hasTrailingUnderscore(identifier) && !isAllowed(identifier)) {
                context.report(node, "Unexpected dangling '_' in '" + identifier + "'.");
            }
        }
    }

    /**
     * Check if variable expression has a underscore at the end
     * @param {ASTNode} node node to evaluate
     * @returns {void}
     * @private
     */
    function checkForTrailingUnderscoreInVariableExpression(node) {
        var identifier = node.id.name;

        if (typeof identifier !== "undefined" && hasTrailingUnderscore(identifier) &&
            !isSpecialCaseIdentifierInVariableExpression(identifier) && !isAllowed(identifier)) {
            context.report(node, "Unexpected dangling '_' in '" + identifier + "'.");
        }
    }

    /**
     * Check if member expression has a underscore at the end
     * @param {ASTNode} node node to evaluate
     * @returns {void}
     * @private
     */
    function checkForTrailingUnderscoreInMemberExpression(node) {
        var identifier = node.property.name,
            isMemberOfThis = node.object.type === "ThisExpression";

        if (typeof identifier !== "undefined" && hasTrailingUnderscore(identifier) &&
            !(isMemberOfThis && allowAfterThis) &&
            !isSpecialCaseIdentifierForMemberExpression(identifier) && !isAllowed(identifier)) {
            context.report(node, "Unexpected dangling '_' in '" + identifier + "'.");
        }
    }

    //--------------------------------------------------------------------------
    // Public API
    //--------------------------------------------------------------------------

    return {
        "FunctionDeclaration": checkForTrailingUnderscoreInFunctionDeclaration,
        "VariableDeclarator": checkForTrailingUnderscoreInVariableExpression,
        "MemberExpression": checkForTrailingUnderscoreInMemberExpression
    };

};

module.exports.schema = [
    {
        "type": "object",
        "properties": {
            "allow": {
                "type": "array",
                "items": {
                    "type": "string"
                }
            },
            "allowAfterThis": {
                "type": "boolean"
            }
        },
        "additionalProperties": false
    }
];