Mocha

When to Use

• Flexibility: You want to pick your assertion library (Chai, Should.js).
• Node.js Backends: standard in many Express/NestJS (legacy) setups.
• Asynchronous Testing: Historically strong support for async (though standard now).

Quick Start

// npm install --save-dev mocha chai
const assert = require("chai").assert;

describe("Array", function () {
  describe("#indexOf()", function () {
    it("should return -1 when the value is not present", function () {
      assert.equal([1, 2, 3].indexOf(4), -1);
    });
  });
});

Core Concepts

BDD Interface

describe

it

before

after

beforeEach

afterEach

Hooks

Async support

async/await

it

it("should save user", async function () {
  const user = new User("tobi");
  await user.save();
});

Best Practices (2025)

• Use

  .only

  and

  .skip

  : Useful during development to run a single test (

  it.only(...)

  ).
• Use

  nyc

  (Istanbul): For code coverage (since Mocha doesn't have it built-in).

• Don't use arrow functions (

  () => {}

  ) if you need

  this.timeout()

  or

  this.slow()

  . Mocha binds the test context to

  this

  .

References

• Mocha Documentation