Testing & diagnostics (intro) in nodejs

In modern Node.js (version 18+), you no longer need external libraries like Jest or Mocha for basic testing. The built-in node:test module provides a fast, native test runner, and node:mock allows you to track or replace function behavior. 1. The Native Test Runner ( node:test ) This module provides the structure for your tests using test or describe/it blocks. import { test, describe, it } from 'node:test'; import assert from 'node:assert'; describe('Math operations', () => {   it('should add two numbers correctly', () => {     assert.strictEqual(1 + 1, 2);   });   test('subtraction works', () => {     assert.strictEqual(5 - 2, 3);   }); }); To run these tests, you simply use the command: node --test 2. Mocking with node:mock Mocks are essential when you want to isolate a piece of code. For example, if you are testing a "Welcome Email" function, you don't want to send a real email every time the test runs. You "mock" the email sender. A. Spies (Tracking function calls) A spy tracks how many times a function was called and with what arguments. import { mock, test } from 'node:test'; import assert from 'node:assert'; test('spying on a function', (t) => {   const sum = (a, b) => a + b;   const mockedSum = t.mock.fn(sum); // Create a tracked version of sum   mockedSum(1, 2);   mockedSum(3, 4);   assert.strictEqual(mockedSum.mock.callCount(), 2);   assert.deepStrictEqual(mockedSum.mock.calls[0].arguments, [1, 2]); }); B. Mocking Objects and Modules You can use mock.method to temporarily change how a specific method on an object behaves. import { mock, test } from 'node:test'; import assert from 'node:assert'; const database = {   save: (data) => { /* real DB logic here */ } }; test('mocking a database save', (t) => {   // Replace 'save' with a fake version that returns true   t.mock.method(database, 'save', () => true);   const result = database.save({ name: 'Tharun' });      assert.strictEqual(result, true);   assert.strictEqual(database.save.mock.callCount(), 1); }); 3. Timers Mocking Testing code that uses setTimeout or setInterval can be slow. Node.js allows you to "fake" time, making a 24-hour delay pass instantly in your test. import { mock, test } from 'node:test'; import assert from 'node:assert'; test('faking time', (t) => {   t.mock.timers.enable(); // Take control of time   let triggered = false;   setTimeout(() => { triggered = true; }, 9999);   t.mock.timers.tick(10000); // Fast-forward 10 seconds   assert.strictEqual(triggered, true); }); 4. Why use the native API? Zero Dependencies: No need to install npm install jest . Your node_modules stays small. Speed: It starts almost instantly because it doesn't need to load a heavy framework. Standardization: Since it's built into Node.js, your tests are more likely to work across different environments without configuration issues. Feature Feature Purpose Test Block test() or it() Defines a single test case. Grouping describe() Organizes related tests. Function Mock mock.fn() Tracks or replaces a standalone function. Method Mock mock.method() Replaces a method on an existing object. Time Control mock.timers Manipulates setTimeout and setInterval.