January 20, 2020

July 31, 2021

Vipin Joshi

Jest as a popular JavaScript testing framework

Introduction

It is the most popular and recommended testing tool for React. It is created by Facebook. Jest is not just a library it is a testing framework. It means it comes with an assertion library, test runner and support for things. As it has been specifically designed to test the React applications, it can also be used on other JavaScript frameworks.

Why Jest

It is simple to set up – it is simple to install on its own. We can just install it directly using either npm or yarn.

How to get started for installation -

• First download node
• Then open node.js command prompt
• To find the version of node we are using Node -v
• Then install npm using command npm install
• Let’s Create a folder in the Desktop named “Jest”
• Then in command prompt use -

cd desktop

cd Jest

Finally, run yarn test or npm run test:

npm run test

Let's get started by writing a test for a hypothetical function that subtracts two numbers. First, create a subtract.js file:

function subtract(a, b)
{
    return a - b;
}
module.exports = subtract;

Then, create a file named subtract.test.js. This will contain our actual test:

const subtract = require('./subtract);
test(subtracts  4 - 2 to equal 3', () =>
{
    expect(sum(4, 2)).toBe(2);
});

Add the following section to your package.json:

{
  "scripts":
}
{
  "test": "jest"
}
"devDependencies":
{
  "jest": "^29.9.0"
}
}

Finally, run yarn or npm run test and Jest will print this message:

PASS ./subtract.test.js

✓ subtracts 4 - 2 to equal 2 (5ms)

Advantages of using Jest

It is compatible: - The framework is compatible with Angular, React, NodeJS, VueJS and other babel-based projects.

Faster than other traditional tools: - It is a very fast testing tool. When our test is CPU bound, it can save significant time from our test runs. The local test used to take 45 minutes Jest dropped it to 14 to 15 minutes.

IT has it all: - It was based on Jasmine, so it has inherited all the good qualities from it. Jest has now been departed from Jasmine, yet they have kept the same functionality but have improved it.

It supports Typescript: - Typescript is a superset of JavaScript that compiles to plain JavaScript. it supports TypeScript via the ts-jest package.

Snapshot testing: - In testing, snapshot testing is a great tool that ensures that your application’s UI does not change unexpectedly between releases.

Timer Mocks: Users can manipulate the Time which is useful. Therefore, the test will not be slow and time can be manipulated by it

• setTimeout()
• setInterval()
• clearTimeout()
• clearInterval()

Write Jest tests to:

• Test a component in isolation
• Test a component’s public API (@api properties and methods, events)
• Test basic user interaction (Like -clicks)
• Verify the DOM output of a component
• Verify that events fire when expected

Conclusion

It is a fast-testing framework. And it requires minimal configuration. It is simple to install. It has good integration with technologies like React, Babel, and typescript. Although it may be considered React –specific test runner but it is compatible and adapts well to any JavaScript library or framework.

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