eslint/lib/rules/no-extra-boolean-cast.js

/**
 * @fileoverview Rule to flag unnecessary double negation in Boolean contexts
 * @author Brandon Mills
 */

"use strict";

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

module.exports = function(context) {

    // Node types which have a test which will coerce values to booleans.
    var BOOLEAN_NODE_TYPES = [
        "IfStatement",
        "DoWhileStatement",
        "WhileStatement",
        "ConditionalExpression",
        "ForStatement"
    ];

    /**
     * Check if a node is in a context where its value would be coerced to a boolean at runtime.
     *
     * @param {Object} node The node
     * @param {Object} parent Its parent
     * @returns {Boolean} If it is in a boolean context
     */
    function isInBooleanContext(node, parent) {
        return (
            (BOOLEAN_NODE_TYPES.indexOf(parent.type) !== -1 &&
                node === parent.test) ||
            // !<bool>
            (parent.type === "UnaryExpression" &&
                parent.operator === "!")
        );
    }


    return {
        "UnaryExpression": function(node) {
            var ancestors = context.getAncestors(),
                parent = ancestors.pop(),
                grandparent = ancestors.pop();

            // Exit early if it's guaranteed not to match
            if (node.operator !== "!" ||
                    parent.type !== "UnaryExpression" ||
                    parent.operator !== "!") {
                return;
            }

            if (isInBooleanContext(parent, grandparent) ||
                // Boolean(<bool>) and new Boolean(<bool>)
                ((grandparent.type === "CallExpression" || grandparent.type === "NewExpression") &&
                    grandparent.callee.type === "Identifier" &&
                    grandparent.callee.name === "Boolean")
            ) {
                context.report(node, "Redundant double negation.");
            }
        },
        "CallExpression": function(node) {
            var parent = node.parent;

            if (node.callee.type !== "Identifier" || node.callee.name !== "Boolean") {
                return;
            }

            if (isInBooleanContext(node, parent)) {
                context.report(node, "Redundant Boolean call.");
            }
        }
    };

};

module.exports.schema = [];