JS: Why am I able to mutate a module in a test and see my changes in my implementation file?

javascript_import_mutations.md

Why am I able to mutate a module in a test and see my changes in my implementation file?

// Test.js

import createHistory from 'history/createBrowserHistory';

describe('something', () => {
  let pushSpy;
  beforeEach(() => {
    pushSpy = jest.fn();
    createHistory = function() {
      return {
        push: pushSpy;
      }
    }
  });

  it('does something', () => {
    subject.doThing();

    // Why does this test pass if I haven't explicitly told Implementation.js
    // to use my mutated copy of createHistory?
    expect(pushSpy).toHaveBeenCalledWith("parameter")
  });
}); 

// Implementation.js

import createHistory from 'history/createBrowserHistory';

export default function doThing() {
    let history = createHistory();
    history.push("parameter")
}

Answer: When you import a module, webpack gives you a reference to the same object every time. This is because webpack caches imports. Therefore changes you make to a module imported in one file will propagate anywhere else you import that module.