Is it possible to set environment variables for exactly one test?

Question

Given something like this:

it('uses env variables correctly', function(done) {
    assert.equal(process.env.x * process.env.y, 10);
});

I would like to set the x and y environment variable for this one test only. Is that possible?

Answer 1

Here is a very common pattern (as answered by Leonid Beschastny).

describe('env', function () {
  let envCache

  // mocking an environment
  before(() => {
    envCache = process.env;
  })

  // running tests
  it('reads env variables correctly', () => {
    process.env.x = 2
    process.env.y = 5

    assert.equal(process.env.x * process.env.y, 10)
  })

  // process.env is not just a plain object
  it('should convert undefined to a string', () => {
    process.env.UNDEF = undefined

    assert.equal(process.env.UNDEF, 'undefined')
  })

  // Trying to restore everything back
  after(() => {
    process.env = env
  })
})

There is a problem here, however. process.env is not a normal object. If the second test is run in isolation, it will pass, as that is how process.env is supposed to work. If instead we run the whole test suite, the second test will fail!

Setting up your tests like that will cause different behaviors based on how you run the test. This is a very bad thing.

Instead you should use delete to clear the values, and then reassign them like so:

describe('env', function () {
  let envCache

  // mocking an environment
  before(() => {
    envCache = process.env;
  })

  // running tests
  it('reads env variables correctly', () => {
    process.env.x = 2
    process.env.y = 5

    assert.equal(process.env.x * process.env.y, 10)
  })

  // process.env is not just a plain object
  it('should convert undefined to a string', () => {
    process.env.UNDEF = undefined

    assert.equal(process.env.UNDEF, 'undefined')
  })

  // Trying to restore everything back
  after(() => {
    Object.keys(process.env).forEach((key) => { delete process.env[key] })
    Object.entries(envCache).forEach(([ key, value ]) => {
      if (key !== undefined) {
        process.env[key] = value
      }
    })
  })
})

This will keep the behavior more consistent, in case someone forgets that you shouldn't assign a key of process.env to be undefined instead of deleting them.