How to stub process.env in node.js?

From my understanding of process.env, you can simply treat it like any other variable when setting its properties. Keep in mind, though, that every value in process.env must be a string. So, if you need a particular value in your test:

   it('does something interesting', () => {
      process.env.NODE_ENV = 'test';
      // ...
   });

To avoid leaking state into other tests, be sure to reset the variable to its original value or delete it altogether:

   afterEach(() => {
       delete process.env.NODE_ENV;
   });

I was able to get process.env to be stubed properly in my unit tests by cloning it and in a teardown method restoring it.

Example using Mocha

const env = Object.assign({}, process.env);

after(() => {
    process.env = env;
});

...

it('my test', ()=> {
    process.env.NODE_ENV = 'blah'
})

Keep in mind this will only work if the process.env is only being read in the function you are testing. For example if the code that you are testing reads the variable and uses it in a closure it will not work. You probably invalidate the cached require to test that properly.

For example the following won't have the env stubbed:

const nodeEnv = process.env.NODE_ENV;

const fnToTest = () => {
   nodeEnv ...
}

With sinon you can stub any variable like this.

 const myObj = {
    example: 'oldValue', 
 };

 sinon.stub(myObj, 'example').value('newValue');

 myObj.example; // 'newValue'

This example is form sinon documentation. https://sinonjs.org/releases/v6.1.5/stubs/

With that knowledge, you can stub any environment variable. In your case it would look like this:

 let stub = sinon.stub(process.env, 'FOO').value('bar');

How to quickly mock process.env during unit testing.

https://glebbahmutov.com/blog/mocking-process-env/

const sinon = require('sinon')
let sandbox = sinon.createSandbox()

beforeEach(() => {
  sandbox.stub(process.env, 'USER').value('test-user')
})

it('has expected user', () => {
  assert(process.env.USER === 'test-user', 'wrong user')
})

afterEach(() => {
  sandbox.restore()
})

But what about properties that might not exist in process.env before the test? You can use the following package and then you will be able to test the not exist env variables.

https://github.com/bahmutov/mocked-env

How to stub process.env in node.js?

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