eslint/lib/rules/no-negated-condition.js

/**
 * @fileoverview Rule to disallow a negated condition
 * @author Alberto Rodríguez
 */
"use strict";

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

module.exports = {
    meta: {
        docs: {
            description: "disallow negated conditions",
            category: "Stylistic Issues",
            recommended: false,
            url: "https://eslint.org/docs/rules/no-negated-condition"
        },

        schema: []
    },

    create(context) {

        /**
         * Determines if a given node is an if-else without a condition on the else
         * @param {ASTNode} node The node to check.
         * @returns {boolean} True if the node has an else without an if.
         * @private
         */
        function hasElseWithoutCondition(node) {
            return node.alternate && node.alternate.type !== "IfStatement";
        }

        /**
         * Determines if a given node is a negated unary expression
         * @param {Object} test The test object to check.
         * @returns {boolean} True if the node is a negated unary expression.
         * @private
         */
        function isNegatedUnaryExpression(test) {
            return test.type === "UnaryExpression" && test.operator === "!";
        }

        /**
         * Determines if a given node is a negated binary expression
         * @param {Test} test The test to check.
         * @returns {boolean} True if the node is a negated binary expression.
         * @private
         */
        function isNegatedBinaryExpression(test) {
            return test.type === "BinaryExpression" &&
                (test.operator === "!=" || test.operator === "!==");
        }

        /**
         * Determines if a given node has a negated if expression
         * @param {ASTNode} node The node to check.
         * @returns {boolean} True if the node has a negated if expression.
         * @private
         */
        function isNegatedIf(node) {
            return isNegatedUnaryExpression(node.test) || isNegatedBinaryExpression(node.test);
        }

        return {
            IfStatement(node) {
                if (!hasElseWithoutCondition(node)) {
                    return;
                }

                if (isNegatedIf(node)) {
                    context.report({ node, message: "Unexpected negated condition." });
                }
            },
            ConditionalExpression(node) {
                if (isNegatedIf(node)) {
                    context.report({ node, message: "Unexpected negated condition." });
                }
            }
        };
    }
};