Jest Testing Tutorial: 5 Easy Steps - Testim Blog (2024)

When it comes to unit testing frameworks for JavaScript, Jest is certainly a serious contender for the #1 spot.

Initially, Jest was created by Facebook specifically for testing React applications. It’s one of the most popular ways of testing React components. Since its introduction, the tool has gained a lot of popularity. This popularity has led to the use of Jest for testing both JavaScript front-end and back-end applications.

In this article, we’ll talk about the ins and outs of Jest to help you get started with testing. Before we get there, though, we’ll offer you a refresher on unit testing and its importance for software quality.

After that, we’ll start covering Jest specifically, explaining:

  • its definition
  • what are its main advantages
  • and some of its most important characteristics

We’ll walk you through a 100% hands-on tutorial on how to get started with Jest. You’ll learn more about the vocabulary associated with Jest testing, like mocks and spies. Also, we’ll cover some of the basics of Jest testing, like using describe blocks and the keywords itand expect. Finally, we’ll take a look at snapshot testing and why it’s particularly useful for front-end testing. Let’s get started!

Jest Testing Tutorial: 5 Easy Steps - Testim Blog (1)

The What and Why of Unit Testing

The topic of software testing can often feel overwhelming. There are just too many types of testing, each operating on a different layer, verifying distinct aspects of the application and offering its unique type of feedback.

Among the myriad types of automated testing, unit testing is often cited as the most important one—see: test automation pyramid. Unit tests verify the smallest parts of your application in complete isolation, ensuring they work as expected. In unit testing, you aren’t allowed to interact with external dependencies—e.g. make an HTTP call—nor generate any kind of side-effect.

As a result of those properties, unit tests are usually:

  • very fast to execute
  • relatively easy to setup, not requiring any elaborate configuration
  • very precise in the feedback they provide

In the scale of automated tests, unit tests sit at the extreme opposite of end-to-end testing. The latter provide less-precise feedback, are generally slower, more fragile, though more realistic. The former are super precise in their feedback, are fast, and typically only fail due to the errors in the code.

However, they are in less realistic, because in real life users don’t interact with units in complete isolation.

To sum it up: unit tests are far from being the only type of tests your application needs, but they should represent a significant portion of your testing strategy.

What Is Jest?

Jest is a popular test framework for JavaScript. It claims to provide “delightful JavaScript testing” and, after our tutorial, I bet you might agree with this claim! Jest prides itself in offering a complete and hassle-free experience.

The completeness comes from the fact that Jest doesn’t rely on third-party tools for much of its functionality, like some competitors do. And the hassle-free part is due to Jest’s zero configuration setup. You can install it and start writing your first test in no time.

As mentioned in the introduction, Jest has gained a lot of popularity over recent years for both front-end and back-end testing. Many large companies—including Twitter, Instagram, Pinterest, and Airbnb—use Jest for React testing.

Jest itself is actually not a library but a framework. There’s even a CLI tool that you can use from the command line. To give an example, the CLI tool allows you to run only specific tests that match a pattern. Besides that, it hosts much more functionality, which you can find in the CLI documentation.

In summary, this means that Jest offers a test runner, assertion library, CLI tool, and great support for different mocking techniques. All of this makes it a framework and not just a library.

Let’s take a quick look at the advantages of Jest.

Advantages of Jest

Here’s a shortlist of Jest advantages.

  1. Offers a CLI tool to control your tests easily
  2. Comes with an interactive mode that automatically runs all affected tests for the code changes you’ve made in your last commit
  3. Provides syntax to test a single test or skip tests with .only and .skip. This feature is useful when debugging individual tests
  4. Provides excellent documentation with plenty of examples and a supportive community. You can join the Jest community via Discord or ask questions on Stack Overflow
  5. Brings easy mocking to developers as it’s one of the most painful things to do for testing engineers. We explain further in this post how Jest mocking works
  6. Jest offers code coverage out of the box through its CLI—just use the –coverage option or the collectCoverage property in the Jest configuration file.

Jest Characteristics

From Jest’s website, we can find four main characteristics of Jest:

  • Zero config: “Jest aims to work out of the box, config free, on most JavaScript projects.” This means you can simply install Jest as a dependency for your project, and with no or minimal adjustments, you can start writing your first test.
  • Isolated: Isolation is a very important property when running tests. It ensures that different tests don’t influence each other’s results. For Jest, tests are executed in parallel, each running in their own process. This means they can’t interfere with other tests, and Jest acts as the orchestrator that collects the results from all the test processes.
  • Snapshots: Snapshots are a key feature for front-end testing because they allow you to verify the integrity of large objects. This means you don’t have to write large tests full of assertions to check if every property is present on an object and has the right type. You can simply create a snapshot and Jest will do the magic. Later, we’ll discuss in detail how snapshot testing works.
  • Rich API: Jest is known for having a rich API offering a lot of specific assertion types for very specific needs. Besides that, its great documentation should help you get started quickly.

Before we dive a bit further into the Jest vocabulary, let’s show you how you can get started with this tool in practice.

Get Started With Jest: A Practical, Hands-On Tutorial in 5 Steps

We’ll now walk you through our five step tutorial on how to get started with testing using Jest.

1. Install Jest Globally

The first step will be to install Jest globally. That way, you gain access to Jest’s CLI. Make sure you have Node.js installed, because you’ll use npm.

Go to your terminal and run the following command:

npm install -g jest

Once the installation is complete, execute jest –version to see the version installed.

2. Create a Sample Project

You’ll now create a npm-based project to house our production code and test code.

Start by creating a folder and accessing it:

mkdir learning-jestcd learning-jest

Then, run npm init -y to create a project. As a result, you should have a package.json file inside your folder, with this content:

{ "name": "learning-jest", "version": "1.0.0", "description": "", "main": "index.js", "scripts": { "test": "echo \"Error: no test specified\" && exit 1" }, "keywords": [], "author": "", "license": "ISC"}

Now, create a file called index.js and paste the following content on it:

function fizz_buzz(numbers) { let result = [] for (number of numbers) { if (number % 15 === 0) { result.push('fizzbuzz') } else if (number % 3 === 0) { result.push('fizz') } else if (number % 5 === 0) { result.push('buzz') } else { result.push(number) } } return result.join(', ')}module.exports = fizz_buzz;

The code above contains a function that solves the famous FizzBuzz programming interview question.

3. Add Jest to the Project

You’ll now add Jest as a dev dependency to the project. Run the following command:

npm install --save-dev jest

Then, go to your package.json file and change this part:

"scripts": { "test": "echo \"Error: no test specified\" && exit 1" },

To this:

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

4. Write Your First Test

Now, create a new file called index.test.js. Paste the following content on it:

const fizz_buzz = require('./index');describe("FizzBuzz", () => { test('[3] should result in "fizz"', () => { expect(fizz_buzz([3])).toBe('fizz'); }); test('[5] should result in "buzz"', () => { expect(fizz_buzz([5])).toBe('buzz'); }); test('[15] should result in "fizzbuzz"', () => { expect(fizz_buzz([15])).toBe('fizzbuzz'); }); test('[1,2,3] should result in "1, 2, fizz"', () => { expect(fizz_buzz([3])).toBe('fizz'); });});

We’ll explain Jest’s syntax in more detail later. For now, understand we’re verifying that:

  • passing an array containing 3 should result in “fizz”
  • an array containing 5 should result in “buzz”
  • an array containing 15 should result in “fizzbuzz”
  • passing an array with 1, 2, and 3 should result in “1, 2, fizz”

5. Run Your First Test

You’re now ready to run your first test. Back to your terminal, simply run npm test. You should see a result like the following:

Jest Testing Tutorial: 5 Easy Steps - Testim Blog (2)

As you can see, all four tests passed. All test suites were executed—which makes sense, since we only have one. The total time for execution was 0.616 seconds.

Now that you had a test of Jest, let’s take a step back and understand, in more detail, its syntax and vocabulary.

Jest Vocabulary

Let’s take a look at two of the most commonly used Jest terms that are also used in other testing tools: mock and spy.

Jest Vocabulary:Mock

From the Jest documentation, we can find the following description for a Jest mock: “Mock functions make it easy to test the links between code by erasing the actual implementation of a function, capturing calls to the function (and the parameters passed in those calls).”

In addition, we can use a mock to return whatever we want it to return. This is very useful to test all the paths in our logic because we can control if a function returns a correct value, wrong value, or even throws an error.

In short, a mock can be created by assigning the following snippet of code to a function or dependency:


Here’s an example of a simple mock, where we just check whether a mock has been called. We mock mockFn and call it. Thereafter, we check if the mock has been called:

const mockFn = jest.fn();mockFn();expect(mockFn).toHaveBeenCalled();

The following example also mocks a return value for checking specific business logic. We mock the returnsTrue function and let it return false:

const returnsTrue = jest.fn(() => false);console.log(returnsTrue()); // false;

Next up, let’s explore what a spy is.

Jest Vocabulary:Spy

A spy has a slightly different behavior but is still comparable with a mock. Again, from the official docs, we read, “Creates a mock function similar to jest.fn() but also tracks calls to object[methodName]. Returns a Jest mock function.”

What this means is that the function acts as it normally would—however, all calls are being tracked. This allows you to verify if a function has been called the right number of times and held the right input parameters.

Below, you’ll find an example where we want to check if the play method of a video returns the correct result but also gets called with the right parameters. We spy on the play method of the video object.

Next, we call the play method and check if the spy has been called and if the returned result is correct. Pretty straightforward! In the end, we must call the mockRestore method to reset a mock to its original implementation.

const video = require('./video');test('plays video', () => {const spy = jest.spyOn(video, 'play');const isPlaying =;expect(spy).toHaveBeenCalled();expect(isPlaying).toBe(true);spy.mockRestore();});

OK, now that we know about the two most used technical terms, let’s explore the basic structure.

Jest Basics

Let’s take a look at some basics on writing tests with Jest.

Jest Basics:Describe Blocks

A describe block is used for organizing test cases in logical groups of tests. For example, we want to group all the tests for a specific class. We can further nest new describe blocks in an existing describe block.

To continue with the example, you can add a describe block that encapsulates all the tests for a specific function of this class.

Jest Basics: “It” or “Test” Tests

Furthermore, we use the test keyword to start a new test case definition. The it keyword is an alias for the test keyword. Personally, I like to use it, which allows for more natural language flow of writing tests. To give an example:

describe('Beverage()', () => { it('should be delicious', () => { expect(myBeverage.delicious).toBeTruthy(); });});

Jest Basics: Matchers

Next, let’s look at the matchers Jest exposes. A matcher is used for creating assertions in combination with the expectkeyword. We want to compare the output of our test with a value we expect the function to return.

Again, let’s look at a simple example where we want to check if an instance of a class is the correct class we expect. We place the test value in the expectkeyword and call the exposed matcher function toBeInstanceOf(<class>) to compare the values. The test results in the following code:

it('should be instance of Car', () => { expect(newTruck()).toBeInstanceOf(Car);});

The complete list of exposed matchers can be found in the Jest API reference.

Jest Basics: Setup and Teardown

It’s important we understand how to prepare and clean up a test. For example, a particular test relies on a mocked database. We don’t want to call a function to set up and clean up the mocked database for each test.

To solve this problem, we can use the beforeEach and afterEach functions to avoid code duplication. Both functions allow you to execute logic before or after each test.

Here’s an example of mocking a database before each test and tear it down when each test has finished.

describe('tests with database', () => { beforeEach(() => { initDB() }) afterEach(() => { removeDB() }) test('if country exists in database', () => { expect(isValidCountry('Belgium')).toBe(true) })})

Moreover, you can also make use of beforeAll and afterAll functions. Both functions will run before or after all tests, but only once. You can use these functions to create a new database connection object and destroy it when you’ve completed the tests.

beforeAll(() => { return createDBConnection()})afterAll(() => { return destroyDBConnection()})

Lastly, let’s take a look at snapshot testing.

Jest Basics: Snapshot Testing for React Front Ends

At last, the Jest documentation suggests using snapshot tests to detect UI changes. As I mentioned earlier, snapshot testing can also be applied for checking larger objects, or even the JSON response for API endpoints.

Let’s take a look at an example for React where we simply want to create a snapshot for a link object. The snapshot itself will be stored with the tests and should be committed alongside code changes.

it('renders correctly', () => { const tree = renderer .create(<Link page="">Facebook</Link>) .toJSON(); expect(tree).toMatchSnapshot();});

Following, the above code renders the following snapshot:

exports[`renders correctly 1`] = `<a className="normal" href="" onMouseEnter={[Function]} onMouseLeave={[Function]}> Facebook</a>`;

If the link object changes, this test will fail in the future. If the changes to the UI elements are correct, you should update the snapshots by storing the results in the snapshot file. You can automatically update snapshots using the Jest CLI tool by adding a “-u” flag when executing the tests.

Getting Started With Jest Testing

Finally, we’ve covered all the basic elements for you to get started with Jest testing. When you’re writing your first test cases, it can feel a bit uncomfortable writing mocks. However, mocks are especially useful in unit testing because they allow you to test the business logic of your function without worrying about its dependencies.

Jest Testing Tutorial: 5 Easy Steps - Testim Blog (3)

If you want to learn more about Jest testing, I suggest reading this guide on unit testing with Jest. I also want to refer you to the Jest cheat sheet. Now go forward and begin with Jest testing!

This post was written by Michiel Mulders. Michiel is a passionate blockchain developer who loves writing technical content. Besides that, he loves learning about marketing, UX psychology, and entrepreneurship. When he’s not writing, he’s probably enjoying a Belgian beer!

Jest Testing Tutorial: 5 Easy Steps - Testim Blog (2024)


How to learn testing in Jest? ›

Get Started With Jest: A Practical, Hands-On Tutorial in 5 Steps
  1. Install Jest Globally. The first step will be to install Jest globally. ...
  2. Create a Sample Project. You'll now create a npm-based project to house our production code and test code. ...
  3. Add Jest to the Project. ...
  4. Write Your First Test. ...
  5. Run Your First Test.
Mar 25, 2022

How to skip all Jest tests? ›

How to ignore test cases in jest? We can use test. skip, describe. skip functions to skip individual and blocks of tests in jest.

What are the weaknesses of Jest? ›

The primary drawbacks of Jest are due to its youth and lack of popularity among JavaScript developers. This kind of technology can be really helpful at times, such as when you can run and debug your tests in an IDE like WebStorm.

How can I make my Jest test faster? ›

As prescribed by Jest, one way to mitigate this issue and improve the speed by up to 50% is to run tests sequentially. Another alternative is to set the max worker pool to ~4. Specifically, on Travis-CI (free plan machines have only 2 CPU cores), this can reduce test execution time in half.

Is Jest the best testing framework? ›

Jest is arguably the most popular JavaScript testing framework used and maintained by Facebook. The JEST testing framework provides a “zero-configuration” testing experience. Jest is a highly preferred framework for applications based on React. It provides a straightforward and very convenient user interface.

What is the difference between Jest test and Jest it? ›

The difference between test and it in Jest

Basically, it is an alias for test , so they are functionally the same. So, what makes it different? This alias is created to make your tests more readable from a test output point of view. It can really help make your tests more readable from a readability point of view.

How do I stop Jest after first fail? ›

Jest Bail — Stop After the First Failing Test

You may configure the bail option to a boolean value or a number. Using a boolean for bail will either stop or not stop after the first failed test. In contrast, you can use a number to be specific about the number of tests before stopping the test run.

Can you do end to end testing with Jest? ›

In Node. js development, you can use a combination of the Chrome API Puppeteer and the JavaScript testing framework Jest to automate e2e testing, allowing you to ensure that the user interface (UI) of your application is still functioning as you fix bugs and add new features.

How to pass mock data in Jest? ›

In order to mock properly, Jest needs jest.mock('moduleName') to be in the same scope as the require/import statement. Here's a contrived example where we have a module that provides a summary of all the files in a given directory. In this case, we use the core (built in) fs module. // `fs` APIs are used.

Is Jest better than Selenium? ›

In general, using a testing framework designed specifically for unit testing, such as Jest or Mocha, is better than using Selenium. These platforms do not require a web browser or web driver, providing a fast, easy, and more robust way to test specific individual classes and functions in isolation.

What are 4 weaknesses? ›

What are some examples of "What are your weaknesses?"
  • Too self-critical.
  • Too critical of other people's work.
  • Difficulty delegating tasks.
  • Disorganized.
  • Too detail-oriented.
  • Need more experience in X.

Is Jest a Selenium? ›

Since Jest is a Selenium test framework and Selenium is built upon Java. So testers have to install the Java Development Kit on the system and then configure the system with the JAVA environment.

Why is test taking so hard? ›

Test anxiety may be caused by a number of factors such as poor test performance in the past, lack of confidence, feelings of extreme pressure or fear of failure, nervousness about having to perform or a number of other problems.

How can I get better at tests? ›

Strategies for Improving Test Performance
  1. Take good notes.
  2. Review notes after each class.
  3. Use SQ3R.
  4. Get control of your time.
  5. Begin reviewing a week before tests.
  6. Test yourself.
  7. Practice good test taking strategies.

Why does Jest take so long? ›

Populating the cache. The first time we run tests in our application, Jest will need to take a bit longer as it can't take advantage of cached data. Jest spends the majority of the first time it runs transpiling TypeScript.

Why is Jest so popular? ›

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.

Is Jest a BDD or TDD? ›

BDD tests are often written using a natural language syntax that makes them more readable and understandable by non-technical team members. Jest supports both TDD and BDD patterns, and you can choose to use either approach based on your preference and the needs of your project.

Does Jest use Selenium? ›

Jest is a JavaScript Testing Framework with a focus on simplicity. Our main focus is using Jest with Selenium. Selenium is a great tool to automate our functional tests on websites and web applications in our favorite language.

Is Jest for frontend or backend? ›

Jest is a JavaScript-based testing framework that lets you test both front-end and back-end applications. Jest is great for validation because it comes bundled with tools that make writing tests more manageable.

What is the difference between Selenium and Jest? ›

Jest belongs to "Javascript Testing Framework" category of the tech stack, while Selenium can be primarily classified under "Browser Testing". "Open source" is the primary reason why developers consider Jest over the competitors, whereas "Automates browsers" was stated as the key factor in picking Selenium.

Who uses Jest? ›

Jest is a Javascript Testing Framework by Facebook. It is used most commonly for unit testing. Unit testing is when you provide input to a unit of code(usually, a function) and match the output with the expected output.

What is the bail in Jest? ›

bail [number | boolean]

By default, Jest runs all tests and produces all errors into the console upon completion. The bail config option can be used here to have Jest stop running tests after n failures. Setting bail to true is the same as setting bail to 1 .

How do I clear my Jest cache? ›

To clear Jest's cache, you can do either of the following:
  1. Use the Jest --clearCache CLI Option;
  2. Locate and Remove the Cache Directory.
Feb 3, 2022

Are Jest tests run sequentially? ›

Order of Execution​

Once the describe blocks are complete, by default Jest runs all the tests serially in the order they were encountered in the collection phase, waiting for each to finish and be tidied up before moving on.

Who should write end-to-end tests? ›

QA team usually writes end-to-end tests using Selenium or a similar framework. They use a browser or a test automation tool to execute end-to-end tests. You should write end-to-end tests in a language that is easy for testers to use.

What is the best testing library for react? ›

Top React Testing Libraries / Frameworks
  • Jest. Jest is the most popular testing framework created and maintained by Facebook. ...
  • Mocha. Mocha is another popular testing framework for Javascript developers. ...
  • Chai. ...
  • Jasmine. ...
  • Karma. ...
  • Enzyme. ...
  • Cypress IO. ...
  • Puppeteer.
Feb 14, 2023

How do I pass a mock exam? ›

The do's and don'ts of mock exams
  1. Do your homework and do it carefully! ...
  2. Do ask questions! ...
  3. Do use your time wisely! ...
  4. Do look back at your notes! ...
  5. Do keep yourself hydrated! ...
  6. Don't stress out! ...
  7. Don't panic and do keep calm! ...
  8. Don't highlight everything!

How to mock 2 API calls in Jest? ›

To mock an API call in a function, you just need to do these 3 steps:
  1. Import the module you want to mock into your test file.
  2. jest. mock() the module.
  3. Use . mockResolvedValue(<mocked response>) to mock the response.
May 29, 2021

How to mock a whole module in Jest? ›

jest.mock(moduleName, factory, options) ​
  1. jest. mock('../moduleName', () => {
  2. return jest.fn(() => 42);
  3. });
  4. // This runs the function specified as second argument to `jest.mock`.
  5. const moduleName = require('../moduleName');
  6. moduleName(); // Will return '42';
Apr 22, 2023

Why use Jest instead of Jasmine? ›

Jest is a more modern and comprehensive testing framework than Jasmine. Jest includes features like snapshot testing, code coverage analysis, and parallel test execution, which are unavailable in Jasmine.

Which is better Jest or cypress? ›

Cypress is built on a new architecture and runs in the same run-loop as the application being tested. As a result Cypress provides better, faster, and more reliable testing for anything that runs in a browser. Cypress works on any front-end framework or website. What is Jest?

Is Jest based on Jasmine? ›

Jest is an open source JavaScript unit testing framework, used by Facebook to test all JavaScript code including React applications. Jest is built on top of Jasmine.

How do you answer the 3 biggest weaknesses? ›

Here are 10 examples of the best weaknesses to mention in a job interview:
  • I focus too much on the details. ...
  • I have a hard time letting go of projects. ...
  • I have trouble saying "no" ...
  • I get impatient with missed deadlines. ...
  • I could use more experience in … ...
  • I sometimes lack confidence. ...
  • I can have trouble asking for help.
Mar 16, 2023

What are your 3 key weaknesses? ›

Sample responses to “What are your weaknesses?”
  • Self-criticism. I can be quite critical of myself, which can lead to negative self-talk and eventual burnout. ...
  • Fear of public speaking. I am a naturally shy person. ...
  • Procrastination. ...
  • Issues with delegating tasks. ...
  • Lack of experience with skill or software.
Jun 15, 2023

How do you answer top 3 weaknesses? ›

Best Weaknesses to Share With an Interviewer
  1. Lack of Patience.
  2. Lack of Organization.
  3. Trouble with Delegation.
  4. Timidity.
  5. Lack of Tactfulness.
  6. Fear of Public Speaking.
  7. Weak Data Analysis Skills.
  8. Indecisiveness.
Apr 27, 2023

Is Jest only for unit testing? ›

Jest is an excellent test runner for frontend code, coupled with enzyme and react-testing-library for React apps, it can be a potent tool to test your components. But here is the catch: it should be used ONLY to write unit tests for isolated components.

Why use Jest for testing? ›

Jest is a JavaScript testing framework designed to ensure correctness of any JavaScript codebase. It allows you to write tests with an approachable, familiar and feature-rich API that gives you results quickly. Jest is well-documented, requires little configuration and can be extended to match your requirements.

Is Jest only for JavaScript? ›

Jest is a JavaScript testing framework built by Facebook and primarily designed for React-based applications, but is also used with Babel, JavaScript, Node, Angular, and Vue.

What is the hardest test to pass? ›

Top 10 Toughest Exams in World To Crack in 2023
  • Gaokao. ...
  • IIT-JEE. ...
  • UPSC. ...
  • Mensa. ...
  • GRE (Graduate Record Examination) ...
  • CFA (Chartered Financial Analyst) ...
  • CCIE (Cisco Certified Internetworking Expert) ...
  • GATE (Graduate Aptitude Test in Engineering, India)
May 19, 2023

How do you get a 100 on a hard test? ›

Follow these study tips to make your best grade!
  1. Get informed. Don't walk into your test unprepared for what you will face. ...
  2. Think like your teacher. ...
  3. Make your own study aids. ...
  4. Practice for the inevitable. ...
  5. Study every day. ...
  6. Cut out the distractions. ...
  7. Divide big concepts from smaller details. ...
  8. Don't neglect the “easy” stuff.

How long is it normal to study for a test? ›

If you have kept a good daily and weekly schedule, 15-20 hours should be about right for a mid-term, 20-30 for a final exam. Major papers take substantially more time and effort.

How can I study for a test in 2 days? ›

At Home:
  1. Organize your notes. Rewrite or type them up so you can actually read what you've written. ...
  2. Review the material. ...
  3. If you don't already have them, make flashcards with a question, term, or vocabulary word on the front of the card, and the answer on the back.
  4. Stay focused!
Aug 11, 2019

What should I do 5 minutes before a test? ›

Make a list of important details. Depending on the subject, make a quick list of important dates, characters, plot points, or formulae. Anything that you can take from memory will be a helpful review in five minutes.

What are 5 testing strategies? ›

However, there are some general test taking strategies that will improve your chances of getting the grade you want on most, if not all, tests.
  1. Listen to the Instructions. ...
  2. Read the Entire Test. ...
  3. Do a “Brain Dump” ...
  4. Answer the Questions You Know First. ...
  5. Answer the Questions You Skipped. ...
  6. Be Sure the Test is Complete.
Sep 29, 2022

What is the disadvantage of Jest? ›

The primary drawbacks of Jest are due to its youth and lack of popularity among JavaScript developers. This kind of technology can be really helpful at times, such as when you can run and debug your tests in an IDE like WebStorm. WebStorm didn't even support running Jest tests till recently.

How do I make my Jest test faster? ›

As prescribed by Jest, one way to mitigate this issue and improve the speed by up to 50% is to run tests sequentially. Another alternative is to set the max worker pool to ~4. Specifically, on Travis-CI (free plan machines have only 2 CPU cores), this can reduce test execution time in half.

How to test function using Jest? ›

  1. 'test' is simply a keyword in Jest. We write tests by using a function provided by Jest called test . ...
  2. 'expect' is also a keyword in Jest. As the name suggests, we expect something from our function or the code we have written. ...
  3. 'matchers' is not a keyword in Jest but is used to call a collection of methods in Jest.
Nov 28, 2019

What is Jest used for in testing? ›

Jest is a JavaScript testing framework designed to ensure correctness of any JavaScript codebase. It allows you to write tests with an approachable, familiar and feature-rich API that gives you results quickly.

What should I test with Jest? ›

Jest is a JavaScript testing framework built by Facebook and primarily designed for React-based applications, but is also used with Babel, JavaScript, Node, Angular, and Vue. It can be used to test NestJS, Next. js, and GraphQL, too.

How to test database using Jest? ›

Testing with Jest
  1. //server. js const app = require('./index. ...
  2. //package.json "scripts": { "start": "node ./bin/www", "test": "jest --config ./jest.config.js --forceExit --coverage --runInBand" }, Lastly create a jest.config.js file at the root of the project. ...
  3. //index. spec.

How to mock an object using Jest? ›

To mock an object in Jest, use the jest. mock() function with the path to the module you want to mock. You can then define a mock implementation for the object's methods and properties using jest. fn().

How do you generate Jest test cases? ›

The fastest way to generate a jest unit test file from a js or ts source. Simply open a source file and select Generate Jest tests from the command palette (cmd + shift + P).

What is the benefit of Jest testing? ›

It is used to test every component and app built around JavaScript, for example - web apps rendering from browsers. Jest also has a massive advantage compared to other popular test automation frameworks because of its non-reliance on third-party apps and software.

How to test an API using Jest? ›

Testing a simple API using jest and supertest. Let's start off building a very simple application – we'll add a database and more routes later, let's just focus on the syntax for now. const express = require("express"); const app = express(); const bodyParser = require("body-parser"); app. use(bodyParser.

How to check type of data in Jest? ›

You can use the matcher to compare instances of a value.
  1. Example: expect(result). ...
  2. Example: expect(result instanceof Date). ...
  3. boolean expect(typeof target). ...
  4. number expect(typeof target). ...
  5. string expect(typeof target). ...
  6. array expect(Array. ...
  7. object expect(target && typeof target === 'object'). ...
  8. null expect(target === null).
Jun 24, 2020

Do tests run in order in Jest? ›

Once the describe blocks are complete, by default Jest runs all the tests serially in the order they were encountered in the collection phase, waiting for each to finish and be tidied up before moving on.

Which command is used to run the Jest test? ›

Running from command line​

You can run Jest directly from the CLI (if it's globally available in your PATH , e.g. by yarn global add jest or npm install jest --global ) with a variety of useful options. If you'd like to learn more about running jest through the command line, take a look at the Jest CLI Options page.


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