How to mock environnement variable with jest and TypeScript

When writing tests for your application, it’s often necessary to mock certain aspects of the environment, such as environment variables. This allows you to test your code in a controlled and predictable manner, without relying on external factors that may change over time.

Jest is a popular JavaScript testing framework that makes it easy to mock environment variables. In this blog post, we’ll show you how to set up a mock environment variable with Jest.

1- Option 1

An easy approach is to create a file named setEnvVars.js, and add the necessary environment variables within it. For instance in “.jest” folder:

// .jest/setEnvVars.ts
process.env.MY_VAR= 'myvarvalue';
process.env.MY_SECRET= 'mysupersecret-value';

The next step is to modify your jest.config.ts file and indicate it to utilize the setEnvVars.ts file.

// jest.config.ts

import type { Config } from 'jest';

const config: Config = {
  setupFiles: ['<rootDir>/jest/setVenVars.ts'],
// ... additional settings
};

export default config;

That’s all! Now when you execute your tests, the code will have access to the mocked environment variables.

2- Option 2

Alternatively, you can use jest-environment-dotenv to handle this for you.

npm install jest-environment-dotenv

and in jest.config.ts

// jest.config.ts
{
  testEnvironment: 'jest-environment-dotenv',
  testEnvironmentOptions: {
    path: '.env.test',
  },
}

In your test files, you will be able to access the environment variables from the process.env object.

That’s it! Your environment variables should now be accessible in your Jest tests.