Install Jest using
Note: Jest documentation uses
yarn commands, but
npm will also work. You can compare
npm commands in the yarn docs, here.
Let's get started by writing a test for a hypothetical function that adds two numbers. First, create a
Then, create a file named
sum.test.js. This will contain our actual test:
Add the following section to your
yarn test or
npm run test and Jest will print this message:
You just successfully wrote your first test using Jest!
This test used
toBe to test that two values were exactly identical. To learn about the other things that Jest can test, see Using Matchers.
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.
Here's how to run Jest on files matching
config.json as a configuration file and display a native OS notification after the run:
If you'd like to learn more about running
jest through the command line, take a look at the Jest CLI Options page.
Based on your project, Jest will ask you a few questions and will create a basic configuration file with a short description for each option:
To use Babel, install required dependencies via
Configure Babel to target your current version of Node by creating a
babel.config.js file in the root of your project:
The ideal configuration for Babel will depend on your project. See Babel's docs for more details.
Making your Babel config jest-aware
Jest will set
'test' if it's not set to something else. You can use that in your configuration to conditionally setup only the compilation needed for Jest, e.g.
babel-jestis automatically installed when installing Jest and will automatically transform files if a babel configuration exists in your project. To avoid this behavior, you can explicitly reset the
Babel 6 support
Jest 24 dropped support for Babel 6. We highly recommend you to upgrade to Babel 7, which is actively maintained. However, if you cannot upgrade to Babel 7, either keep using Jest 23 or upgrade to Jest 24 with
babel-jest locked at version 23, like in the example below:
While we generally recommend using the same version of every Jest package, this workaround will allow you to continue using the latest version of Jest with Babel 6 for now.
Jest supports TypeScript, via Babel. First, make sure you followed the instructions on using Babel above. Next, install the
@babel/preset-typescript to the list of presets in your
However, there are some caveats to using TypeScript with Babel. Because TypeScript support in Babel is purely transpilation, Jest will not type-check your tests as they are run. If you want that, you can use ts-jest instead, or just run the TypeScript compiler tsc separately (or as part of your build process).
You may also want to install the
@types/jest module for the version of Jest you're using. This will help provide full typing when writing your tests with TypeScript.
@types/*modules it's recommended to try to match the version of the associated module. For example, if you are using
@types/jestis ideal. In general, try to match the major (
26) and minor (
4) version as closely as possible.