According to Wikipedia, test-driven development (TDD) is a software development process that converts software requirements into test cases before the software is actually developed. The steps of the TDD cycle are as follows:

  1. Add a test (the new test should fail)
  2. Write the simplest code that makes the new test pass (YAGNI)
  3. Refactor the code

TDD Cycle

So, in order to set up a Node.js project for TDD, we need a way to run unit tests on the code that will be developed for it. There are two popular tools for doing this: Mocha is a popular JavaScript test framework and Chai is an assertion library that can be nicely paired with it.

We can get started by launching a bash shell and running the following commands to initialize such a project.

mkdir tdd-project
cd tdd-project
npm init -y
npm install --save-dev mocha
npm install --save-dev chai

The tdd-project directory should now have a new file called package.json which contains the following:

{
  "name": "tdd-project",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "keywords": [],
  "author": "",
  "license": "ISC",
  "devDependencies": {
    "chai": "^4.3.4",
    "mocha": "^9.1.3"
  }
}

Now all we have to do is modify the scripts property in this file. We could simply make the test script run the mocha command. However, mocha by default only finds test files with extensions .js, .mjs, and .cjs in a directory named test. On the other hand, the following script finds all test files ending with .test.js in all directories except node_modules.

{
  "name": "tdd-project",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "mocha \"./{,!(node_modules)/**/}*.test.js\""
  },
  "keywords": [],
  "author": "",
  "license": "ISC",
  "devDependencies": {
    "chai": "^4.3.4",
    "mocha": "^9.1.3"
  }
}

After adding the test script, we can then run all tests by calling npm test in the project directory. To get started with writing test files, we create a new file somewhere within the project (not in the node_modules directory of course) named sample.test.js. Then we add the following contents:

const expect = require('chai').expect;

describe('sample', () => {
    it('fails this test', () => {
        expect(true).to.equal(false);
    });
});

Running npm test should now return the following:


> tdd-project@1.0.0 test
> mocha "./{,!(node_modules)/**/}*.test.js"



  sample
    1) fails this test


  0 passing (2ms)
  1 failing

  1) sample
       fails this test:

      AssertionError: expected true to equal false
      + expected - actual

      -true
      +false

      at Context.<anonymous> (sample.test.js:5:19)
      at processImmediate (node:internal/timers:464:21)

Congratulations on writing your first failing test! I have placed all the code in this post in a Github repository for your convenience: https://github.com/miguelamezola/tdd-project.git