Usually, JavaScript Unit tests are run in the frontend or on the browser. Here, some javascript code is written for testing and this can be done for a specific module in an application or for a page in a website depending upon the need, and then this code is combined with HTML as an inline event handler. These unit tests are organized as different suites. Each and every suite contains a number of tests particularly performed for a separate module and above all, they don’t arise any conflicts with any other module and runs with fewer dependencies.

JSUnit, Unit.js, QUnit, Jasmine, Karma, Mocha, Jest, Tape, AVA etc are some tools and frameworks that are being used to perform JavaScript Unit Testing. Among them, I am going to talk about a javascript unit testing framework called Jest – a delightful javascript testing

Jest

Jest is a JavaScript unit testing framework maintained by Facebook. Though it is often used for testing React components it can also be used for testing any javascript code. Some of the important strength it exhibits are Jest is fast, and it can perform snapshot testing, and optional features. It provides the user a zero-configuration testing experience which is one of the features all programmers welcomes. For jest to find and run your test you need to place your sample tests in __tests__ folder or give a name to your test files with extension as .specs.js or .test.js.

Jest Installation

Jest installation using yarn:

yarn add --dev jest

Or using npm:

npm install --save-dev jest

If you use create-react-app then you don’t need to install Jest because it is automatically installed in it.

Jest test Example

Let’s look for a test that will multiply two numbers and test it using jest:
Add the following lines to your package.json:

{
"scripts": {
"test": "jest"
}
}

Start by writing a test function that multiplies two numbers. First, create a multiply.js file:

function multiply(x, y) {
return x * y;
}
module.exports = multiply;

Then, create a file let it be multiply.test.js. Now, this is where our actual test occur:

const multiply = require('./multiply');
test('Multiplying 2 * 3 equal to 6', () => {
expect(multiply(2, 3)).toBe(6);
});

Run yarn test and Jest will run on all test files and returns the following end result:

PASS ./multiply .test.js
? Multiplying 2 * 3 equal to 6 (7ms)

Congrats, now you just successfully wrote your first jest test 🙂

Jest used in:-
Jest is used to test various web applications, APIs, node.js services, and mobile apps. Facebook, Twitter, The New York Times, Spotify, Instagram etc. are some among them.

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