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WTF is Jest?

Learn the basics of the JavaScript testing framework, Jest.

Jest is a JavaScript testing framework.

It's JavaScript, so yeah, there are a whole heck of a lot of frameworks to choose from when it comes to writing tests. Many feel passionate about one over another. But Jest seems to be the more popular choice today.

As a test framework, its job is to provide you with an API that will help you test your program's code. Here's a quick example:

Let's say you have a function called add that is exported from a utils.js file. It adds to numbers together and returns the result. It looks like this:

utils.js

exports.add = function (num1, num2) {
return num1 + num2
}

You may have a Jest file right alongside it that loads the function and writes tests for it. A simple test would be to ensure that 1 + 1 = 2. That Jest file might look like this:

__tests__/utils.js

const { add } = require("../utils")

test("1 + 1 = 2", () => {
expect(add(1, 1)).toEqual(2)
})
note

Notice I named the file the same and put it in a __tests__ directory. This is a convention expected by Jest.

You can alternatively group the files together in the same directory and call it utils.spec.js and utils.test.js. Both will get picked up and run by Jest by default, unless you've changed the configuration.

This was a super simple example just to give you a preview. To actually put this to action and learn more about Jest, visit its Getting Started guide.

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