How to implement unit tests in Node.js?

Unit testing is a method of software testing that verifies the individual parts of the software. Here is how you can implement unit tests in Node.js:

1. Choose a testing framework: Some popular options include Mocha, Jest or Jasmine. For our example, we will use Mocha and Chai.

1. Install Mocha and Chai: You can install Mocha and Chai via NPM:
```
npm install —save-dev mocha chai
```

1. Create a test file: Once installed, you can create a new file test.js. This is where you will write your tests.

1. Write a Test: Tests are structured through two levels of functions describe, and it.

Below is an example of a simple test for an addNumbers function:
```
let assert = require(‘chai’).assert;
let addNumbers = require(‘../functions’).addNumbers;

describe(‘addNumbers()’, function() { it(‘should add two numbers’, function() { let result = addNumbers(5, 7); assert.equal(result, 12); });
});
```

1. Running Tests: Mocha looks for the test folder by default, so as long as you’ve put your tests there, you don’t have to specify the files to be tested. You can run your tests with the Mocha command:

```
./node_modules/mocha/bin/mocha
```

1. Hooks: Mocha provides hooks(built-in test case functions) which can be used to perform setup and teardown operations. They include before(), after(), beforeEach(), and afterEach().

```
describe(‘Hooks’, function() {

//Before hook before(function() { // runs once before the first test in this block }); //After hook after(function() { // runs once after the last test in this block }); //BeforeEach hook beforeEach(function() { // runs before each test in this block }); //AfterEach hook afterEach(function() { // runs after each test in this block }); }); ```

1. Async testing: You can also test asynchronous code with either callbacks, promises or async/await.

Remember, for a unit test, you should follow these principles.

- A unit test should test functionality in isolation.
- Mock the database and network methods, if any.
- Use a predictable amount of resources.
- Repeatable and independent of the environment.