✅ JavaScript Unit Testing: Start Simple But Grow Very Powerful

If you’ve ever written code and wondered “How do I know this works every time?”, you’re already thinking about unit testing.

Unit testing is the practice of writing small, automated tests to check that individual pieces of code (like a function) behave exactly the way you expect. In JavaScript, this can be as simple as testing a basic function, or as advanced as mocking dependencies and testing Promises.

In this lesson, we’ll explore both worlds:

Basic Assertions → The foundation of testing.
Unit Testing Promises → Using modern tools like Mocha, Chai, and Sinon.

🧪 Basic Assertions

At its simplest, a unit test is just an assertion:

“If I run this code, do I get the result I expect?”

Let’s build a tiny custom assertion function:

function assert(outcome, description) {
  var passFail = outcome ? "pass" : "fail";
  console.log(passFail, ":", description);
  return outcome;
};

This function prints pass if the test condition is true, or fail if it’s false.

Now let’s test a simple add function:

function add(num1, num2) {
  return num1 + num2;
}

A failing test (wrong expectation):

var result = add(5, 20);
assert(result == 24, "add(5, 20) should return 25...");
// Output: fail : add(5, 20) should return 25...

A corrected test:

assert(result == 25, "add(5, 20) should return 25...");
// Output: pass : add(5, 20) should return 25...

👉 A real-world test suite would include multiple assertions — testing positive numbers, negative numbers, and even edge cases like add(0, 0) — to make sure your function works in all scenarios.

⚡ Testing Promises with Modern Tools

JavaScript isn’t just about simple functions — modern apps rely heavily on Promises for asynchronous work. Testing them requires a bit more setup.

Popular testing tools:

Mocha → A test runner (organizes and runs tests).
Chai → An assertion library (human-readable checks).
chai-as-promised → Extends Chai to handle Promises easily.
Sinon → For spies, stubs, and mocks.
Proxyquire → To override dependencies (inject stubs).

🔹 Testing a Function that Returns a Promise

Imagine a ping function that returns a Promise:

const ping = () => {
  return new Promise((resolve, _reject) => {
    const response = processResponse(data);
    resolve(response);
  });
};

We don’t want the real processResponse — we just want to test ping in isolation.

So we use:

proxyquire → To replace processResponse with a fake one (formattingStub).
sinon.stub() → To define what the stub should return.

In the test, we assert:

expect(pingResult).to.be.fulfilled;
expect(pingResult).to.eventually.equal(response);

Thanks to chai-as-promised, this reads almost like English: “I expect this Promise to be fulfilled and eventually equal the response.”

🔹 Testing a Function that Consumes a Promise

Now let’s test a wrapper that calls ping:

const pingWrapper = () => {
  ping.then((response) => {
    // do something with the response
  });
};

Steps for testing:

Stub the ping function itself with proxyquire.
Use sinon-stub-promise to control how the stubbed Promise behaves:
```
pingStub.returnsPromise().resolves(response);
```
After running pingWrapper, assert that ping was called properly:
```
expect(pingSpy).to.have.been.calledWith(response);
```

👉 This approach ensures that your tests don’t rely on real external dependencies, making them fast, reliable, and isolated.

✅ Summary

Unit testing in JavaScript can start simple but grow very powerful:

Basic Assertions → Use functions like assert() or libraries like Chai to check expected results.
Testing Promises → Requires stubbing dependencies and using tools like Mocha, Chai-as-Promised, Sinon, and Proxyquire.
Good tests → Small, focused, reliable, and independent of real-world services.

The more you test, the more confidence you’ll have that your code won’t break when users interact with it.

📝 Review – Fill in the Gaps

A unit test checks a small, discrete piece of __.
The assert function returns “pass” if the test outcome is __.
add(5, 20) should correctly return __.
Mocha is a JavaScript test __.
Chai is used as an __ library.
To test Promises, Chai can be extended with chai-______.
Sinon is useful for creating spies, stubs, and __.
Proxyquire allows you to override a module’s __.
expect(pingResult).to.eventually.equal(response); checks the resolved value of a __.
A good test suite should cover not just one case but multiple __.

🐞 Debugging Techniques in JavaScript: Techniques To Understanding Your Program

⚡ Evaluating JavaScript: Showing Both The Power And The Dangers Of eval() And Its Alternatives