Jest: A Unit Testing Framework

The story of testing frameworks and TypeScript

Over the years, there have been many unit testing frameworks that have been built for JavaScript. Some of the oldest, and therefore most mature, frameworks include Jasmine, QUnit and Mocha.

There have also been attempts to write unit testing frameworks in TypeScript, including MaxUnit or tsUnit, but these frameworks never really took off.

When using a unit testing framework, one of the most important things to consider is the range of testing features that it has:

• Does it have the ability to raise errors, for example?

• Can it mock out dependencies easily, such as an asynchronous call to a REST server or returning rows from a database?

• Can it allow for special testing code to replace a function call completely?

• Can it test throwing exceptions, and can it test rendered DOM elements?

The ease with which TypeScript can integrate with JavaScript libraries means we can use a fully featured JavaScript unit testing framework as if it were written in TypeScript.

Most testing frameworks have a similar suite of features, including a watch mode that will detect changes to test files and automatically re-run tests.

One of the popular testing frameworks currently available is Jest, which was written by the Facebook team and integrates with Babel, TypeScript, Node, React, Angular, and Vue.

Introduction to Jest

Jest is a simple-to-configure and powerful JavaScript unit ...