test process.env with Jest

Question

I have an app that depends on environmental variables like:

const APP_PORT = process.env.APP_PORT || 8080;

and I would like to test that for example:

• APP_PORT can be set by node env variable.
• or that an express app is running on the port set with process.env.APP_PORT

How can I achieve this with Jest? Can I set these process.env variables before each test or should I mock it somehow maybe?

Answer 1

The way I did it can be found in this SO question.

It is important to resetModules before each test and then dynamically import the module inside the test:

describe('environmental variables', () => {
  const OLD_ENV = process.env;

  beforeEach(() => {
    jest.resetModules() // this is important
    process.env = { ...OLD_ENV };
    delete process.env.NODE_ENV;
  });

  afterEach(() => {
    process.env = OLD_ENV;
  });

  test('will receive process.env variables', () => {
    // set the variables
    process.env.NODE_ENV = 'dev';
    process.env.PROXY_PREFIX = '/new-prefix/';
    process.env.API_URL = 'https://new-api.com/';
    process.env.APP_PORT = '7080';
    process.env.USE_PROXY = 'false';

    const testedModule = require('../../config/env').default

    // ... actual testing
  });
});

Answer 2

Depending on how you can organize your code, another option can be to put the env variable within a function that's executed at runtime.

In this file, the env var is set at import time, and requires dynamic requires in order to test different env vars (as described in this answer):

const env = process.env.MY_ENV_VAR;

const envMessage = () => `MY_ENV_VAR is set to ${env}!`;

export default myModule;

In this file, the env var is set at envMessage execution time, and you should be able to mutate process.env directly in your tests:

const envMessage = () => {
  const env = process.env.MY_VAR;
  return `MY_ENV_VAR is set to ${env}!`;
}

export default myModule;

Jest test:

const vals = [
  'ONE',
  'TWO',
  'THREE',
];

vals.forEach((val) => {
  it(`Returns the correct string for each ${val} value`, () => {
    process.env.MY_VAR = val;

    expect(envMessage()).toEqual(...