Adding Jest tests to a project
Tags: testing
Published: 2022-04-02 05:14:24

Now that we are talking about testing, let's try out Jest, a pretty well-used testing framework for JavaScript.

I've been using Jest for the past couple of months, finding it really powerful so far.

There are some other really great frameworks out there, and choosing one comes down to flavor and needs.

For this article, I'll use a simple JavaScript project as our basis.
It will have a determination script to determine if certain statements are true or false.

You can download the starting point from [GitHub](https://github.com/rebelchris/determination).

## Validating our function

You can see the `determine.js` function in the main folder.
This is basically our app. It's used to determine if a variable is a number.

In the most basic case, it should perform the following tests:

```js
isNumber(123);
// Should be true

isNumber('abc');
// Should be false
```

We first have to add Jest to our project to make this testable.

```bash
npm install -D jest
```

The code above will install the jest package in our dev dependencies.

Now we should modify our `package.json` to include a testing command.

```js
"scripts": {
	"test": "jest"
}
```

The cool thing about Jest is that it can fire automatically on any files it deems testable.

By default, it can run on the following files:

- `.js` files inside `__tests__` folders
- `.test.js` files inside your project
- `.spec.js` files inside your project

For now, let's add a `determine.test.js` file. This makes sense as we have our `determine.js` file as our basis.

To start the file, we have to import our function first.

```js
const { isNumber } = require('./determine');
```

Then we can define tests. Each test is an isolated scope that can pass or fail.

Let's start by adding a test made to expect the right answer.

```js
test('Validate a number', () => {
  expect(isNumber(1)).toBeTruthy();
});
```

We start by defining the test, which holds a string (the name of the test) and executes a function.
The actual test states:

Expect -> function (variables) -> to be true

I really love how Jest made these super human-readable and understandable.

We ask for the function `isNumber` to be true when we pass the number one to it.

We can now run the test by invoking the following command:

```bash
npm run test
```

![Running tests in Jest](https://cdn.hashnode.com/res/hashnode/image/upload/v1648020995444/EBaHEHPDm.png)

And as you can see, our test is succeeding.

Let's step up our test and add a failing test, but leave the same evaluation.

```js
test('Invalidate a string', () => {
  expect(isNumber('ABC')).toBeTruthy();
});
```

We use the same test but pass a wrong number, so the test should fail.

![Failing tests](https://cdn.hashnode.com/res/hashnode/image/upload/v1648021096941/g0OjfSukA.png)

And it does fail, which is expected!
So let's modify the test case to evaluate a false value.

```js
test('Invalidate a string', () => {
  expect(isNumber('ABC')).toBeFalsy();
});
```

Our test will succeed, but we still check for a failed value!

The completed test can be found here on [GitHub](https://github.com/rebelchris/determination/tree/test).

## Conclusion

Although this is a very straightforward test, it can be super important. Let's say we modify something in this `isNumber` function. Our test will now quickly show something went wrong.

I'll be writing a bit more about writing some particular test cases and showing you how you can use and leverage Jest.

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