Set Up the Project:

npm init -y

Install Dependencies:

npm install typescript jest ts-jest @types/jest --save-dev

Create a tsconfig.json:

{
 "compilerOptions": {
   "target": "ES6",
   "module": "CommonJS",
   "strict": true,
   "outDir": "./dist",
   "esModuleInterop": true
 },
 "include": ["src/merge.ts", "test/merge.test.ts"]
}

Create a jest.config.js:

module.exports = {
 preset: 'ts-jest',
 testEnvironment: 'node',
};

Add the test script in package.json:

"scripts": {
 "test": "jest"
}

Execute the tests:

npm test