node/tools/node_modules/eslint/lib/rules/no-duplicate-case.js

/**
 * @fileoverview Rule to disallow a duplicate case label.
 * @author Dieter Oberkofler
 * @author Burak Yigit Kaya
 */

"use strict";

//------------------------------------------------------------------------------
// Requirements
//------------------------------------------------------------------------------

const astUtils = require("./utils/ast-utils");

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

/** @type {import('../shared/types').Rule} */
module.exports = {
    meta: {
        type: "problem",

        docs: {
            description: "Disallow duplicate case labels",
            recommended: true,
            url: "https://eslint.org/docs/latest/rules/no-duplicate-case"
        },

        schema: [],

        messages: {
            unexpected: "Duplicate case label."
        }
    },

    create(context) {
        const sourceCode = context.sourceCode;

        /**
         * Determines whether the two given nodes are considered to be equal.
         * @param {ASTNode} a First node.
         * @param {ASTNode} b Second node.
         * @returns {boolean} `true` if the nodes are considered to be equal.
         */
        function equal(a, b) {
            if (a.type !== b.type) {
                return false;
            }

            return astUtils.equalTokens(a, b, sourceCode);
        }
        return {
            SwitchStatement(node) {
                const previousTests = [];

                for (const switchCase of node.cases) {
                    if (switchCase.test) {
                        const test = switchCase.test;

                        if (previousTests.some(previousTest => equal(previousTest, test))) {
                            context.report({ node: switchCase, messageId: "unexpected" });
                        } else {
                            previousTests.push(test);
                        }
                    }
                }
            }
        };
    }
};