node/tools/eslint/lib/rules/eqeqeq.js

/**
 * @fileoverview Rule to flag statements that use != and == instead of !== and ===
 * @author Nicholas C. Zakas
 */

"use strict";

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

module.exports = function(context) {

    /**
     * Checks if an expression is a typeof expression
     * @param  {ASTNode} node The node to check
     * @returns {boolean} if the node is a typeof expression
     */
    function isTypeOf(node) {
        return node.type === "UnaryExpression" && node.operator === "typeof";
    }

    /**
     * Checks if either operand of a binary expression is a typeof operation
     * @param {ASTNode} node The node to check
     * @returns {boolean} if one of the operands is typeof
     * @private
     */
    function isTypeOfBinary(node) {
        return isTypeOf(node.left) || isTypeOf(node.right);
    }

    /**
     * Checks if operands are literals of the same type (via typeof)
     * @param {ASTNode} node The node to check
     * @returns {boolean} if operands are of same type
     * @private
     */
    function areLiteralsAndSameType(node) {
        return node.left.type === "Literal" && node.right.type === "Literal" &&
                typeof node.left.value === typeof node.right.value;
    }

    /**
     * Checks if one of the operands is a literal null
     * @param {ASTNode} node The node to check
     * @returns {boolean} if operands are null
     * @private
     */
    function isNullCheck(node) {
        return (node.right.type === "Literal" && node.right.value === null) ||
                (node.left.type === "Literal" && node.left.value === null);
    }

    /**
     * Gets the location (line and column) of the binary expression's operator
     * @param {ASTNode} node The binary expression node to check
     * @param {String} operator The operator to find
     * @returns {Object} { line, column } location of operator
     * @private
     */
    function getOperatorLocation(node) {
        var opToken = context.getTokenAfter(node.left);
        return {line: opToken.loc.start.line, column: opToken.loc.start.column};
    }

    return {
        "BinaryExpression": function(node) {
            if (node.operator !== "==" && node.operator !== "!=") {
                return;
            }

            if (context.options[0] === "smart" && (isTypeOfBinary(node) ||
                    areLiteralsAndSameType(node)) || isNullCheck(node)) {
                return;
            }

            if (context.options[0] === "allow-null" && isNullCheck(node)) {
                return;
            }

            context.report(
                node, getOperatorLocation(node),
                "Expected '{{op}}=' and instead saw '{{op}}'.",
                {op: node.operator}
            );
        }
    };

};