How to mock/replace getter function of object with Jest?

You could use Object.defineProperty

Object.defineProperty(myObj, 'prop', {
  get: jest.fn(() => 'bar'),
  set: jest.fn()
});

For anyone else stumbling across this answer, Jest 22.1.0 introduced the ability to spy on getter and setter methods.

Edit: like in scieslak's answer below, because you can spy on getter and setter methods, you can use Jest mocks with them, just like with any other function:

class MyClass {
  get something() {
    return 'foo'
  }
}

jest.spyOn(MyClass, 'something', 'get').mockReturnValue('bar')
const something = new MyClass().something

expect(something).toEqual('bar')