Install Mocha and Sinon.js:

npm install mocha sinon

Create the test directory and file:

mkdir test && touch test/file.js

Import assert and sinon:

// test/file.js
const assert = require('assert');
const sinon = require('sinon');

Now create a test case that stubs exit from process:

describe('process.exit', () => {
  it('is stubbed', () => {
    sinon.stub(process, 'exit');
    process.exit(1);
    assert(process.exit.isSinonProxy);
  });
});

Write your assertions with assert:

assert(process.exit.called);
assert(process.exit.calledWith(1));
assert.equal(process.exit.args[0][0], 1);

Or write them with sinon:

sinon.assert.called(process.exit);
sinon.assert.calledWith(process.exit, 1);

Also, don’t forget to restore the original function when the test is done:

it('is stubbed', () => {
  // ...
  process.exit.restore();
});

A good refactor tip is to move the stub and restore logic to mocha’s before and after hooks:

describe('process.exit', () => {
  before(() => {
    sinon.stub(process, 'exit');
  });

  after(() => {
    process.exit.restore();
  });

  it('is stubbed', () => {
    process.exit(1);
    assert(process.exit.isSinonProxy);
    sinon.assert.called(process.exit);
    sinon.assert.calledWith(process.exit, 1);
  });
});

Lastly, you can alter the stub’s behavior with callsFake:

it('is stubbed and faked', () => {
  process.exit.callsFake(() => {
    return 'foo';
  });
  assert.equal(process.exit(), 'foo');
});

To run the tests in your command-line:

npx mocha

Check out the documentation for more information.