In this lesson, we’ll get the most fundamental understanding of what an automated test is in JavaScript. A test is code that throws an error when the actual result of something does not match the expected output. 

Tests can get more complicated when you’re dealing with code that depends on some state to be set up first (like a component needs to be rendered to the document before you can fire browser events, or there needs to be users in the database). However, it is relatively easy to test **pure functions** (functions which will always return the same output for a given input and not change the state of the world around them).

Fundamentals of Testing in JavaScript

Throw an Error with a Simple Test in JavaScript

We have a bug in the `sum` function. It's doing subtraction instead of addition. We could easily fix this, but let's go ahead and write an automated test that can make sure that this bug never surfaces again.

An automated test in JavaScript is code that throws an error when things are unexpected. Let's do that. Let's get our `result` from the `sum` of three and seven. 

#### simple.js
```javascript
const result = sum(3, 7)
```

We will say our `expected` is 10. We can say if the `result` is not equal to the `expected` value, then we can throw a new error that says, "Result is not equal to expected."

```javascript
const expected = 10
if (result !== expected){
    throw new Error(`${result} is not equal to ${expected}`)
}
```

To run this, we can run `node lessons/simple.js`. 

We will get our error, "-4 is not equal to 10." 

![Error](http://res.cloudinary.com/dg3gyk0gu/image/upload/v1543907671/transcript-images/javascript-write-the-simplest-test-in-javascript-error.png)

If we replace this with addition and run that again, our script passes without throwing errors. This is the most fundamental form of a test.

```javascript
const sum = (a, b) => a + b
```

Let's go ahead and add another test for `subtract`. I am going to change this from `const` to `let`. 

```javascript
const subtract = (a, b) => a - b
```
We will say, `result` is now equal to subtract of seven and three, and our `expected` is now equal to four."

```javascript
result = subtract(7, 3)
expected = 4
```

We will just copy and paste this here. 

```javascript
if (result !== expected){
    throw new Error(`${result} is not equal to ${expected}`)
}
```
Save that and run our script again. It passes without error. In review, this is the most fundamental form of a test in JavaScript. It's simply a code that will throw an error when the result is not what we expect.

Let's go ahead and break this again. We'll run that script again. We're getting an error message. The Javascript testing framework is to make that error message as useful as possible so we can quickly identify what the problem is and fix it.

Do you know what a testing framework does? Do you know what makes a testing framework different from a testing or assertion library? The best way to use a tool effectively is to understand how it works. And the best way to understand how a tool works is by making it yourself! 

In this short course, we’ll learn how testing frameworks and assertion libraries work by building our own, simple version of each.

Intro to Fundamentals of Testing in JavaScript

Let’s add a simple layer of abstraction in our simple test to make writing tests easier. The assertion library will help our test assertions read more like a phrase you might say which will help people understand our intentions better. It will also help us avoid unnecessary duplication.

Abstract Test Assertions into a JavaScript Assertion Library

One of the limitations of the way that this test is written is that as soon as one of these assertions experiences an error, the other tests are not run. It can really help developers identify what the problem is if they can see the results of all of the tests.

Let’s create our own `test` function to allow us to encapsulate our automated tests, isolate them from other tests in the file, and ensure we run all the tests in the file with more helpful error messages.

Encapsulate and Isolate Tests by building a JavaScript Testing Framework

Our testing framework works great for our synchronous test. What if we had some asynchronous functions that we wanted to test? We could make our callback functions `async`, and then use the `await` keyword to wait for that to resolve, then we can make our assertion on the `result` and the `expected`.

Let’s make our testing framework support promises so users can use async/await.


Support Async Tests with JavaScripts Promises through async await

These testing utilities that we built are pretty useful. We want to be able to use them throughout our application in every single one of our test files.

Some testing frameworks provide their helpers as global variables. Let’s implement this functionality to make it easier to use our testing framework and assertion library. We can do this by exposing our `test` and `expect` functions on the global object available throughout the application.

Provide Testing Helper Functions as Globals in JavaScript

Up to this point we’ve created all our own utilities. As it turns out, the utilities we’ve created mirror the utilities provided by Jest perfectly! Let’s install Jest and use it to run our test!

Throw an Error with a Simple Test in JavaScript

Transcript

Throw an Error with a Simple Test in JavaScript