Testing modules that use env variables in `jest`

01-example.ts

const isDevelopment = process.env.NODE_ENV !== 'production'

export function log(...args) {
  if (isDevelopment) {
    console.log(...args)
  }
}

export type Example = 'hello'

02-example.spec.ts

// You can import types safely, as they get stripped out in the parsed code
import { Example } from './01-example';

let OLD_ENV: string | undefined;

beforeAll(() => {
  // Store the actual value so it can be restored after all test cases finish
  OLD_ENV = process.env.NODE_ENV;
  // You can also set a default value here for all tests, if you want:
  // process.env.NODE_ENV = 'default'
  // NOTE: in newer versions of `@types/node`, process.env is a Readonly object so TypeScript will complain
  // You may need to add `@ts-ignore` above the assignment in such cases.
});

beforeEach(() => {
  // Clear jest cache
  jest.resetModules();
});

afterAll(() => {
  process.env.NODE_ENV = OLD_ENV;
});

it('logs stuff in development', async () => {
  // set value for this test case
  process.env.NODE_ENV = 'development';

  // import the module
  const { log } = await import('./01-example');
  // good old require would work too, but we want to preserve type safety!
  // const { log } = require('./example')

  const spy = jest.spyOn(console, 'log');
  log('hello');
  expect(spy).toHaveBeenCalledWith('hello');
});

it('does not log stuff in production', async () => {
  process.env.NODE_ENV = 'production';

  const { log } = await import('./01-example');

  const spy = jest.spyOn(console, 'log');
  log('hello');
  expect(spy).not.toHaveBeenCalled();
});