How to mock import.meta.env?

[koenwoortman]

In my code I have a reference to import.meta.env.MODE. Can we mock this value in a test case?

Such that I could run a test as if the value of import.meta.env.MODE was X.

[sheremet-va]

import.meta.env.MODE = 'X'

[koenwoortman]

Tried this, but is doesn't work as I would expect for references in my code, only in the test files.

Lets for example say I have the following:

// ./src/mode.js
export const mode = import.meta.env.MODE;

And to test the above use the following:

// ./tests/mode.spec.js
import { it, expect } from 'vitest'
import { mode } from '../src/mode'

it('...', () => {
  import.meta.env.MODE = 'X'
  expect(mode).toBe('X')
});

I would expect this test to pass. However, import.meta.env.MODE from ./src/mode.js returns "test" instead of "X"

 FAIL  tests/mode.spec.js > ...
AssertionError: expected 'test' to be 'X' // Object.is equality
 ❯ tests/mode.spec.js:6:15
      4| it('...', () => {
      5|        import.meta.env.MODE = 'X'
      6|        expect(mode).toBe('X')
       |               ^
      7| });
      8| 

  - Expected   "X"
  + Received   "test"

[sheremet-va]

Of course it would be, primitives are accessed by value, not by reference in JavaScript. You defined mode when import.meta.env.MODE was defined as test.

You need to import module after you redefined it:

import.meta.env.MODE = 'x'
const { mode } = import('../src/mode')
expect(mode).toBe('X')

[sheremet-va]

If you for some reason have a module that exports all env like that, it would be easier to use vi.mock to mock full module:

import env from './envs'
vi.mock('./envs', () => ({ mode: 'X' }))
// you can also redefined it with
vi.spyOn(env, 'mode', 'get').mockReturnValue('NEW')

[koenwoortman]

@sheremet-va thanks!