Mocha

Mocha is a flexible JavaScript test framework. Unlike Jest, it does not come with an assertion library, mocking, or snapshotting built-in. It is a runner that allows you to choose your own tools (usually Chai + Sinon).

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 (suite), it (test), before, after, beforeEach, afterEach.

Hooks

Mocha allows defining hooks at the root level or inside suites to set up/tear down environments.

Async support

Use async/await in the it block callback.

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

Best Practices (2025)

Do:

• 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:

• Don’t use arrow functions (() => {}) if you need this.timeout() or this.slow(). Mocha binds the test context to this.

References

• Mocha Documentation