Configuring Jest for Typescript Unit Tests

Jest is a unit testing library for JavaScript. Testing JavaScript code with Jest is straightforward and doesn't require any additional configuration. When testing Typescript code without compiling it to JavaScript first, a few configurations are required. There are two ways for setting up Jest for Typescript unit test; via Babel or ts-jest. Babel is a popular multipurpose transpiler for JavaScript. ts-jest is an extension of the Typescript compiler tailored to work with Jest. In this article, we will explore and demonstrate how to configure Jest to test Typescript code via ts-jest.

To create a Typescript and Jest testing environment, we will execute the following sequence of activities:

1. Create an initialized working directory.
2. Install Typescript.
3. Configure Typescript.
4. Install Jest and its related dependencies.
5. Configure Jest for Typescript.
6. Edit package.json test script.
7. Try it out.

Let us start by creating an npm-initialized working directory.

1. Create an initialized working directory.

In this step, we will create a directory called jest-ts-tests and initialize it as a node project directory.

Create a directory named ts-jest-env and run

npm init 

to generate package.json file

By executing the above instructions, we now have a directory ready for use. Let us go and install Typescript in the next step.

2. Install Typescript

In the previous step, we created a directory for us to work on. In this step, we will install Typescript as a development dependency as follows.

Run npm i -D typescript

We have successfully installed Typescript, let us configure it in the next step.

3. Configure Typescript

We will install ready-to-use Typescript base configurations instead of starting from scratch. In this article, we will use Typescript base configurations for node version 20. If you are using a different version of node, visit the tsconfig bases docs to find out what you need. You can also create a Typescript configuration file with settings that work for you.

Run the following command to install base Typescript configurations for node version 20.

npm i -D @tsconfig/node20

Create a tsconfig.json file and paste the following configs.

{
  "extends": "@tsconfig/node20/tsconfig.json",
  "$schema": "https://json.schemastore.org/tsconfig",
  "display": "Node 20",
  "_version": "20.1.0",

  "compilerOptions": {
    "lib": ["es2023"],
    "module": "node16",
    "target": "es2022",

    "strict": true,
    "esModuleInterop": true,
    "skipLibCheck": true,
    "forceConsistentCasingInFileNames": true,
    "moduleResolution": "node16",
    "outDir": "./build",
    "rootDir": "./src"
  },
  "include": ["src/**/*.ts"]
}

4. Install Jest and related dependencies.

We are now ready to write Typescript code. In this step, we will install Jest and all the necessary dependencies related to it.

Run the following command to install Jest and the other dependencies.

npm i -D jest ts-jest @types/jest @jest/globals

We need ts-jest to compile Typescript for Jest and @jest/globals to provide Jest global global utilities for typescript.

5. Configure Jest for Typescript

Here, we tell Jest to execute .ts files with ts-jest. To complete this configuration, we will generate a Jest configuration file with ts-jest presets using the following command.

npx ts-jest config:init

Executing the above command generates a jest.config.js file with ts-jest as a preset.

Well done, let us edit our npm test scripts in the next step.

6. Edit package.json test script.

We want to be able to run tests easily by using a simple npm command. To enable the command, edit the scripts object of package.json as follows:

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

The setting above allows us to run tests using the npm test command. The --watch flag tells Jest to run tests every time our code changes.

Next, we proceed to the actual testing to find and verify our configurations.

7. Try it out.

Up to this point, we have installed all the required dependencies for testing Typescript with Jest. Here, we will write a simple function and test it to verify that our configurations are on point.

Create an index.ts file and export a simple function that you want to test. Do not finish the implementation of the function until you write a failing test.

For example: A sum function,

export const sum(num1: number, num2: number): number =>{

}

Create index.test.ts and import sum.

index.test.ts

import {sum} from `index`

Import Jest globals.

index.test.ts

import {sum} from `index`
import { describe, test, expect } from "@jest/globals"

Write test.

index.test.ts

import {sum} from `index`
import { describe, test, expect } from "@jest/globals"

describe('Sum function', () =>{
    test('Returns correct value', () =>{
        expect(sum(2, 3)).toEqual(5)
    })
})

Run the following command to begin your tests:

npm test

Edit your sum function until your test passes.

Congratulations! That's the end of configuring Jest for Typescript unit tests. We have created a working test environment for testing Typescript code using Jest and ts-jest. The resultant directory is available on this GitHub repository. If you are interested in learning other ways of testing typescript code with Jest, feel free to visit the document on using Jest with typescript.