Jest - mock individual function from es6 class - unit-testing

Question

I would like to test whether a particular function is called from the function I'm testing however both of these functions are within the es6 class I am testing.
Setup:
// MyClass.js
export default class MyClass {
constructor () { ... }
foo() {
...
bar()
}
bar() {
...
}
}
So I am testing foo() but as part of my test I want to ensure that bar() is called. I can see how I can do this if bar() was an external dependency as I could just mock that but as it's part of the class I am testing I can't mock the entire class otherwise I wouldn't be calling a concrete implementation in my test.
I was hoping this would've done the trick (as per https://facebook.github.io/jest/docs/en/es6-class-mocks.html) but to no avail:
// MyClass.spec.js
jest.unmock('./MyClass')
import MyClass from './MyClass'
...
test('my test', () => {
var mockBar = jest.fn()
MyClass.bar = mockBar
var myClass = new MyClass()
myClass.foo()
expect(mockBar).toHaveBeenCalled()
})
I also tried mocking the method on the instantiated version of MyClass (i.e. myClass.bar = mockBar) but neither gave the desired results.
Is this mocking structure possible with jest?

Answer 1

When testing specific method of ES6 class (not class instance) you should mock not the property of the class, but property of this class prototype.
jest.unmock('./MyClass')
import MyClass from './MyClass'
...
test('my test', () => {
var mockBar = jest.fn()
MyClass.prototype.bar = mockBar // here's the difference
var myClass = new MyClass()
myClass.foo()
expect(mockBar).toHaveBeenCalled()
})
This is because ES6 classes is just sugar over plain Javascript prototype model, and
class MyClass {
bar() {}
}
is equivalent not to
var MyClass = function () {
MyClass.bar = function bar() {};
return MyClass;
}();
but to
var MyClass = function () {
MyClass.prototype.bar = function bar() {};
return MyClass;
}();