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

/**
 * @fileoverview Rule to require or disallow yoda comparisons
 * @author Nicholas C. Zakas
 */
"use strict";

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

/**
 * Determines whether an operator is a comparison operator.
 * @param {String} operator The operator to check.
 * @returns {boolean} Whether or not it is a comparison operator.
 */
function isComparisonOperator(operator) {
    return (/^(==|===|!=|!==|<|>|<=|>=)$/).test(operator);
}

/**
 * Determines whether an operator is an equality operator.
 * @param {String} operator The operator to check.
 * @returns {boolean} Whether or not it is an equality operator.
 */
function isEqualityOperator(operator) {
    return (/^(==|===)$/).test(operator);
}

/**
 * Determines whether an operator is one used in a range test.
 * Allowed operators are `<` and `<=`.
 * @param {String} operator The operator to check.
 * @returns {boolean} Whether the operator is used in range tests.
 */
function isRangeTestOperator(operator) {
    return ["<", "<="].indexOf(operator) >= 0;
}

/**
 * Determines whether a non-Literal node is a negative number that should be
 * treated as if it were a single Literal node.
 * @param {ASTNode} node Node to test.
 * @returns {boolean} True if the node is a negative number that looks like a
 *                    real literal and should be treated as such.
 */
function looksLikeLiteral(node) {
    return (node.type === "UnaryExpression" &&
        node.operator === "-" &&
        node.prefix &&
        node.argument.type === "Literal" &&
        typeof node.argument.value === "number");
}

/**
 * Attempts to derive a Literal node from nodes that are treated like literals.
 * @param {ASTNode} node Node to normalize.
 * @returns {ASTNode} The original node if the node is already a Literal, or a
 *                    normalized Literal node with the negative number as the
 *                    value if the node represents a negative number literal,
 *                    otherwise null if the node cannot be converted to a
 *                    normalized literal.
 */
function getNormalizedLiteral(node) {
    if (node.type === "Literal") {
        return node;
    }

    if (looksLikeLiteral(node)) {
        return {
            type: "Literal",
            value: -node.argument.value,
            raw: "-" + node.argument.value
        };
    }

    return null;
}

/**
 * Checks whether two expressions reference the same value. For example:
 *     a = a
 *     a.b = a.b
 *     a[0] = a[0]
 *     a['b'] = a['b']
 * @param   {ASTNode} a Left side of the comparison.
 * @param   {ASTNode} b Right side of the comparison.
 * @returns {boolean}   True if both sides match and reference the same value.
 */
function same(a, b) {
    if (a.type !== b.type) {
        return false;
    }

    switch (a.type) {
        case "Identifier":
            return a.name === b.name;

        case "Literal":
            return a.value === b.value;

        case "MemberExpression":

            // x[0] = x[0]
            // x[y] = x[y]
            // x.y = x.y
            return same(a.object, b.object) && same(a.property, b.property);

        case "ThisExpression":
            return true;

        default:
            return false;
    }
}

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

module.exports = {
    meta: {
        docs: {
            description: "require or disallow \"Yoda\" conditions",
            category: "Best Practices",
            recommended: false
        },

        schema: [
            {
                enum: ["always", "never"]
            },
            {
                type: "object",
                properties: {
                    exceptRange: {
                        type: "boolean"
                    },
                    onlyEquality: {
                        type: "boolean"
                    }
                },
                additionalProperties: false
            }
        ]
    },

    create: function(context) {

        // Default to "never" (!always) if no option
        var always = (context.options[0] === "always");
        var exceptRange = (context.options[1] && context.options[1].exceptRange);
        var onlyEquality = (context.options[1] && context.options[1].onlyEquality);

        /**
         * Determines whether node represents a range test.
         * A range test is a "between" test like `(0 <= x && x < 1)` or an "outside"
         * test like `(x < 0 || 1 <= x)`. It must be wrapped in parentheses, and
         * both operators must be `<` or `<=`. Finally, the literal on the left side
         * must be less than or equal to the literal on the right side so that the
         * test makes any sense.
         * @param {ASTNode} node LogicalExpression node to test.
         * @returns {Boolean} Whether node is a range test.
         */
        function isRangeTest(node) {
            var left = node.left,
                right = node.right;

            /**
             * Determines whether node is of the form `0 <= x && x < 1`.
             * @returns {Boolean} Whether node is a "between" range test.
             */
            function isBetweenTest() {
                var leftLiteral, rightLiteral;

                return (node.operator === "&&" &&
                    (leftLiteral = getNormalizedLiteral(left.left)) &&
                    (rightLiteral = getNormalizedLiteral(right.right)) &&
                    leftLiteral.value <= rightLiteral.value &&
                    same(left.right, right.left));
            }

            /**
             * Determines whether node is of the form `x < 0 || 1 <= x`.
             * @returns {Boolean} Whether node is an "outside" range test.
             */
            function isOutsideTest() {
                var leftLiteral, rightLiteral;

                return (node.operator === "||" &&
                    (leftLiteral = getNormalizedLiteral(left.right)) &&
                    (rightLiteral = getNormalizedLiteral(right.left)) &&
                    leftLiteral.value <= rightLiteral.value &&
                    same(left.left, right.right));
            }

            /**
             * Determines whether node is wrapped in parentheses.
             * @returns {Boolean} Whether node is preceded immediately by an open
             *                    paren token and followed immediately by a close
             *                    paren token.
             */
            function isParenWrapped() {
                var tokenBefore, tokenAfter;

                return ((tokenBefore = context.getTokenBefore(node)) &&
                    tokenBefore.value === "(" &&
                    (tokenAfter = context.getTokenAfter(node)) &&
                    tokenAfter.value === ")");
            }

            return (node.type === "LogicalExpression" &&
                left.type === "BinaryExpression" &&
                right.type === "BinaryExpression" &&
                isRangeTestOperator(left.operator) &&
                isRangeTestOperator(right.operator) &&
                (isBetweenTest() || isOutsideTest()) &&
                isParenWrapped());
        }

        //--------------------------------------------------------------------------
        // Public
        //--------------------------------------------------------------------------

        return {
            BinaryExpression: always ? function(node) {

                // Comparisons must always be yoda-style: if ("blue" === color)
                if (
                    (node.right.type === "Literal" || looksLikeLiteral(node.right)) &&
                    !(node.left.type === "Literal" || looksLikeLiteral(node.left)) &&
                    !(!isEqualityOperator(node.operator) && onlyEquality) &&
                    isComparisonOperator(node.operator) &&
                    !(exceptRange && isRangeTest(context.getAncestors().pop()))
                ) {
                    context.report(node, "Expected literal to be on the left side of " + node.operator + ".");
                }

            } : function(node) {

                // Comparisons must never be yoda-style (default)
                if (
                    (node.left.type === "Literal" || looksLikeLiteral(node.left)) &&
                    !(node.right.type === "Literal" || looksLikeLiteral(node.right)) &&
                    !(!isEqualityOperator(node.operator) && onlyEquality) &&
                    isComparisonOperator(node.operator) &&
                    !(exceptRange && isRangeTest(context.getAncestors().pop()))
                ) {
                    context.report(node, "Expected literal to be on the right side of " + node.operator + ".");
                }

            }
        };

    }
};