Upgrade ESLint Configuration for a Mixed JS/TS Project

Problem/Feature Description

An internal tools project has been using ESLint v7 with a .eslintrc.json file. The team is long overdue for an upgrade to ESLint v9 with the modern flat config system. The project is a mix of JavaScript utilities and TypeScript modules, and the old config used @typescript-eslint/eslint-plugin with some TypeScript-specific rules alongside base JavaScript rules.

The team is cautious about this migration — they've heard of projects breaking when rule behavior changes unexpectedly. They want the migration to preserve the spirit of the old config, and they want any style or rule adjustments made separately after the structural migration is stable.

Produce the updated linting setup. Document the migration in a MIGRATION.md file. Do NOT run npm install — just produce the updated config and package files. The grader will read the output files directly.

Output Specification

• Updated package.json — with updated ESLint and plugin devDependencies for ESLint v9
• eslint.config.js or eslint.config.mjs — the new flat config
• MIGRATION.md — a brief description of the migration steps taken and any rule parity decisions

Input Files

The following files are provided as inputs. Extract them before beginning.

=============== FILE: package.json =============== { "name": "internal-tools", "version": "3.0.0", "type": "module", "scripts": { "lint": "eslint . --ext .js,.ts", "lint:fix": "eslint . --ext .js,.ts --fix" }, "devDependencies": { "eslint": "^7.32.0", "@typescript-eslint/eslint-plugin": "^5.62.0", "@typescript-eslint/parser": "^5.62.0" } }

=============== FILE: .eslintrc.json =============== { "parser": "@typescript-eslint/parser", "plugins": ["@typescript-eslint"], "extends": [ "eslint:recommended", "plugin:@typescript-eslint/recommended" ], "rules": { "no-unused-vars": "off", "@typescript-eslint/no-unused-vars": ["error", { "argsIgnorePattern": "^_" }], "no-console": "warn" }, "overrides": [ { "files": ["*.ts"], "rules": { "@typescript-eslint/explicit-function-return-type": "warn" } } ] }

=============== FILE: src/utils.js =============== export function formatDate(date) { return date.toISOString().split('T')[0] }

=============== FILE: src/parser.ts =============== export function parseNumber(value: string): number { const n = parseInt(value, 10) if (isNaN(n)) throw new Error(Invalid number: ${value}) return n }