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.

Fundamentals of Testing in JavaScript

Abstract Test Assertions into a JavaScript Assertion Library

We're pulling `sum` and `subtract` from this math module, and `sum` has a bug.

#### assertion-library.js
```javascript
const {sum, subtract} = require('../math')
```

We can reveal that bug with a simple test.

```javascript
result = sum(3, 7)
expected = 10
if (result !== expected) {
  throw new Error(`${result} is not equal to ${expected}`)
}
```

When we run this file with node, that assertion is throwing an error.

![Sum Error](http://res.cloudinary.com/dg3gyk0gu/image/upload/v1543907669/transcript-images/node-js-build-a-javascript-assertion-library-sum-error.png)

Also, this code is pretty imperative. It'd be nice to write a little abstraction to make it read a little nicer. Let's go ahead and write a simple abstraction to encapsulate this assertion. I'll create a function down here called `expect`. That's going to accept an `actual`.

```javascript
function expect(actual){

}
```

We are going to return an object that has some assertions on it. Our first one here is going to be `toBe`, and that's going to take an `expected` value.

```javascript
function expect(actual) {
  return {
    toBe(expected) {
    }
  }
}
```

We're going to say if the `actual` is not equal to the `expected` value, then we'll `throw a new Error` that says, "Actual is not equal to expected."

```javascript
function expect(actual) {
  return {
    toBe(expected) {
      if (actual !== expected) {
        throw new Error(`${actual} is not equal to ${expected}`)
      }
    }
  }
}
```

We can take this duplicated code, remove that and replace it with:

```javascript
result = sum(3, 7)
expected = 10
expect(result).toBe(expected)
```

If we run that now, we are going to get the same error message. This function is like an assertion library. Fundamentally, it takes an actual value, and then it returns an object that has functions for different assertions that we can make on that actual value.

Here, we have `toBe`. We could also have a `twoEqual`, now it'd take an `expected` value, and maybe do a deep equality check.

We could also have a `toBeGreaterThan` or `toBeLessThan`, and then it could take an `expected` value. There're all kinds of assertions that we could add to our little assertion library here to make writing our test a little easier.

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

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).

Throw an Error with a Simple Test in JavaScript

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!

Abstract Test Assertions into a JavaScript Assertion Library

Transcript

Abstract Test Assertions into a JavaScript Assertion Library