An important basic concept in testing is isolation. You should only test one method at a time, and your tests for one function should not depend upon an external function behaving correctly - especially if that function is being tested elsewhere. The main reason for this is that when your tests fail, you want to be able to narrow down the cause of this failure as quickly as possible. If you have a test that depends on several functions, it can be hard to tell exactly what is going wrong.

Learning Outcomes

By the end of this lesson, you should be able to do the following:

  • Explain what tightly coupled code is
  • Describe a pure function and how it relates to TDD
  • Explain what mocking is

Pure Functions

There are many benefits to using TDD when you write your code. One of the biggest benefits is less obvious at first - it helps you to write better code. If you look back at some of your early projects you will probably notice how tightly coupled everything is. All of your functions include references to functions in other parts of your code, and the whole thing is filled with DOM methods or console.log().

Tightly coupled code is hard to test! Imagine trying to write tests for a function like this:

function guessingGame() {
  const magicNumber = 22;
  const guess = prompt('guess a number between 1 and 100!');
  if (guess > magicNumber) {
    alert('YOUR GUESS IS TOO BIG');
  } else if (guess < magicNumber) {
  } else if (guess == magicNumber) {
    alert('YOU DID IT! 🎉');

Making this testable requires us to split up all the different things that are happening. First, we do not need to test the functions prompt and alert because they are built in to the browser. They are external to our program and whoever wrote them has already tested them. What we do need to test is the number logic, which is much easier if we untangle it from the other functions:

function evaluateGuess(magicNumber, guess) {
  if (guess > magicNumber) {
    return 'YOUR GUESS IS TOO BIG';
  } else if (guess < magicNumber) {
  } else if (guess == magicNumber) {
    return 'YOU DID IT! 🎉';

function guessingGame() {
  const magicNumber = 22;
  const guess = prompt('guess a number between 1 and 100!');
  const message = evaluateGuess(magicNumber, guess);


In this example, the only thing we really need to test is the evaluateGuess function, which is much easier to test because it has a clear input and output and doesn’t call any external functions. This implementation is much nicer as well because it’s much easier to extend. If we wanted to switch out the prompt and alerts for methods that manipulate the DOM we can do that more simply now and if we want to make our game more advanced by letting the user make multiple guesses, that is also easier.

If we had written this program with TDD it is very likely that it would have looked more like the second example to begin with. Test driven development encourages better program architecture because it encourages you to write Pure Functions.


There are two solutions to the ‘tightly coupled code’ problem. The first, and best option is to simply remove those dependencies from your code as we did above, but that is simply not always possible. The second option is mocking - writing “fake” versions of a function that always behaves exactly how you want. For example, if you’re testing a function that gets information from a DOM input, you really don’t want to have to set up a webpage and dynamically insert something into the input just to run your tests. With a mock function, you could just create a fake version of the input-grabbing function that always returns a specific value and use THAT in your test.


  1. If you haven’t already, watch the ‘mocking’ videos from this series.
  2. Too much mocking can be a bad thing. It is sometimes necessary, but if you have to set up an elaborate system of mocks to test any bit of your code, that means your code is too tightly coupled. These two articles (one and two) might be a little extreme, but they contain several really good points about program architecture and testing.
  3. Now that you have some practice and context for TDD, this section of the Jest docs will probably make good sense to you.
  4. Jest includes some really handy mocking functions. Read about them in the official docs.
  5. Watch this amazing video that covers what to test in your codebase. The video is specifically about testing the Ruby language, but that doesn’t matter at all. The concepts here ring true in any language, and luckily Ruby is a clear enough language that you will be able to follow along just fine.
  6. And finally, if you wish, you can add Jest to your webpack setup. Read about that process here.

Knowledge Check

This section contains questions for you to check your understanding of this lesson. If you’re having trouble answering the questions below on your own, review the material above to find the answer.

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