After many years of JavaScript existence, the world is experiencing an advancement of its front- end testing. The tools involved interchange depending on the framework in use. Being categorical when it comes to the choice of framework limits the available testing alternatives. Different frameworks come with varying portability options, and this gives developers the opportunity to discover distinct tools for every different project.

JavaScript comes with three fundamental testing layers. These are test frameworks, assertion libraries, and test runners. Developers use these layers during each testing arrangement. However, there are times when they keep them in a central area for the purpose of forming an individual tool. Jasmine, a development framework which is behavior driven, is a testing library but also acts as an assertion library. Developers have to analyze all these fundamental elements and consider incorporating extra essential tools to their evaluation stacks. These include code inclusion tools and mocking libraries.

This article covers various terms and popular tools used for JavaScript testing of the React framework.

Choosing a JavaScript test runner

A testing stack consists of various parts, with the test runner being the most essential. Newbie developers who are yet to master an organized method of test running may end up doing it in a bulky manner, which can be complex and hard to tackle. This is certain to result in disinterest, or even abandonment of the project. This denies them an opportunity to realize the benefits of assertion libraries and frameworks.

Developers who explore and include Karma in their testing stack have a better experience. Karma can automatically execute various tests in multiple or individual browsers, depending on user preference. It comes in handy to prevent portal quirks from using the developer’s code, while exuding exclusive determination. There are various ways of configuring Karma. Individuals can configure it to ensure standardization of frameworks, plugins, and browsers.

In addition, developers can override the devtool using remote services in order to launch sourcemaps functionality. These remote services include Browserstack and Sauce Labs. While some developers prefer Karma, many developers dealing with react choose to work with Mocha because it is easy to use and more convenient. Executing Mocha tests within a Node.js surrounding is simple. However, this can be complex for developers when testing component synergy which are focused on the DOM or rendering. They may have to integrate JSDOM or Enzyme for better performance. The development world is still waiting for an invention that will allow browser test running with the use of Mocha CLI.

How to choose

Use Karma for analyzing cross browser or significant DOM handling. Developers who do not fancy Karma can use framework’s CLI, in conjunction with extra libraries such as Enzyme and JSDOM. The convenience with Karma is that it makes sure code functionality takes place in the code execution browsers.

Choosing a JavaScript test framework

Test frameworks ensure sufficient organization of tests. They also alert the developers with results which come in either stylized HTML or command-line output. Test frameworks should be capable of executing the before and after code tests/test suite whenever needed, reinforcing asynchronous code and asynchronous testing, and making specific test immobilization easy.

While Jasmine, Mocha, and Jest have the fundamental specifications, they vary a lot and this can be confusing. Jasmine contains assertion libraries known as matchers. They are capable of rendering independent assertion libraries such as Should.js and Chai functionless. Jasmine comes in handy in ensuring adequate invocation of functions, and simulates certain roles which enable developers to reduce dependency trees from a test’s scope.

The functions take charge of almost every usable case, and developers can access them from Jasmine’s spies. Developers will sometimes require established simulation systems, complete with simulation functionality and assertion libraries. These include Sinon.js. While Jasmine plays an important role in maintaining organization within the testing stack, developers will at some point incorporate dynamic libraries to boost performance.

Facebook uses Jest as their test framework. It bases its design on React and plays an essential role when it comes to fundamental testing. However, it is way above the rest thanks to its snapshot testing.

Snapshot testing allows developers to regularize and store the component’s output. It is capable of recognizing any changes that may occur to the output. Users can view the changes, and either accept or decline them. A snapshot is a system’s state during a specific moment. Snapshots are usually stored in the source control and marked for subsequent test runs. Jest comprises the fundamental simulation and assertions functionality. However, just as is the case with Jasmine, developers will at some point require extra reinforcement to boost performance.

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Mocha is distinctive, and remains one of the most concrete frameworks. Mocha cannot execute fancy tests and neither does it have assertion libraries. For this reason, developers using Mocha should include simulation and assertion libraries. Despite this, Mocha is quite dynamic in the functions it performs.

How to Choose

Mocha comes in handy for developers in separately choosing individual sections of a stack. Of course other test frameworks will allow developers to include different simulation and assertion libraries. Many developers may find the process a little complex. The process requires developers to ensure the library they settle for is well spread out within the entire group.

Jasmine or Jest comes in handy for developers with little inclination for obscure requirements. While Jest is a universal JavaScript testing framework, testing snapshot is exclusive to React components. Both Jasmine and Jest are compatible with React. However, developers may want to opt for Jasmine when using varying front-end frameworks.

Choosing a JavaScript assertion library

Assertion libraries are instinctive and pluggable. Developers may have to decide the best that suits their preferences, but developers using Jest or Jasmine do not have to choose an assertion library. In the development industry, developers will want to try new assertion libraries every once in awhile.

Chai allows users to try three divergent assertions which are: should, expect, and assert. While every one of them varies, developers like using Chai because it includes common styles. With so much convenience comes the worry of having varying tests with varying styles, but similar characteristics.

 How to choose

Since Jasmine and Jest come with inbuilt assertions, it is better to continue using them without any alterations. Mocha users may want to add extra assertions to enhance functionality. Chai, on the other hand, can be overpowering at times. Developers can get through this by selecting a different library depending on their preferences.

Choosing JavaScript testing add-ons

Regardless of the test framework that developers choose, they will need to include various extra tools to boost efficient React component testing. Tools which enhance shallow rendering of React components with minimal requirement of DOM are quite dynamic.

Shallow rendering defines the act of rendering only one level depth of components. Nested components in this case do not undergo replacement. They linger around the tree which accumulates the nested components, and hence boost the speed and functionality of tests.

Developers can choose between two options when it comes to assertions, and rendering of the components. They can use react-test-utils in assertions, and react-test-render when it comes to shallow rendering. While this functions well, some developers may want to try a different option such as Enzyme, which is capable of shallow rendering components and restoring any objects which contain jQuery-like API.


Sinon.js is a dynamic simulation library which comes with comprehensive assertions. Developers cannot go wrong in including it to their testing stack. Mocha, Chai, and Karma can play a role in a testing stack, but developers can choose to create a strong foundation with Enzyme and Jest.

Have you got any questions or suggestions? Get in touch with us through commenting below.

Oprettet 18 november, 2017


Software Developer

Lucy is the Development & Programming Correspondent for She is currently based in Sydney.

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