Mock process.env in Jest

31 October 2023

[jest]

Here's how to mock out process.env in a Jest test.

This essentially from this StackOverflow post, reproduced here for my own reference.

The Concept

It turns out that you can set process.env to the value you want, not just read from it. What we'll want to do is save a version of the original process.env object, then set the value we want for our test, then reset it to the original when we're done.

A Source File

Let's say I have a function that relies on process.env for its work. A contrived example:

export function myFunction() {
  return process.env.NEXT_PUBLIC_ENV  === 'test' ? 'testValue' : 'prodValue'
}

A Test Suite

describe('myFunction', () => {
  const ORIG_ENV = process.env

  beforeEach(() => {
    jest.resetModules()
    process.env = { ...ORIG_ENV }
  })

  afterEach(() => {
    process.env = ORIG_ENV
  })

  test('region, with override, use override', () => {
    process.env.NEXT_PUBLIC_ENV = 'test'

    expect(myFunction()).toEqual('testValue')
  })
})

The call to jest.resetModules is to make sure that the setting of process.env doesn't leak out into other tests.

From the jest docs, jest.resetModules:

Resets the module registry - the cache of all required modules. This is useful to isolate modules where local state might conflict between tests.

Set and unset. Noodles, don't noodles. process.env is yours.