How can I use Jest to spy on a method call?

The key is using jests spyOn method on the object's prototype. It should be like this:

const spy = jest.spyOn(Component.prototype, 'methodName');const wrapper = mount(<Component {...props} />);wrapper.instance().methodName();expect(spy).toHaveBeenCalled();

As found here e.g.: Test if function is called react and enzyme

Please note it is also best practice to clear the spied function after each test run

let spyafterEach(() => {  spy.mockClear()})

https://facebook.github.io/jest/docs/en/jest-object.html#jestclearallmocks

I know its a bit late, but I came across this and would suggest that to test componentDidMount initiates the call to your nested method that your test should look something like:

Module

componentDidMount() {  if (this.props.initOpen) {    this.methodName();  }}

Test - Good

it('should call methodName during componentDidMount', () => {    const methodNameFake = jest.spyOn(MyComponent.prototype, 'methodName');    const wrapper = mount(<MyComponent {...props} />);    expect(methodNameFake).toHaveBeenCalledTimes(1);});

If you call componentDidMount then the assertion that methodName was called via componentDidMount is more valid.

Test - Bad

it('should call methodName during componentDidMount', () => {    const spy = jest.spyOn(Component.prototype, 'methodName');    const wrapper = mount(<Component {...props} />);    wrapper.instance().methodName();    expect(spy).toHaveBeenCalled();}

By writing the test like this - you call the method and then assert that it was called. Which of course it will have given you just called it.

If you're trying to test public methods being called on componentDidMount (if you're using TypeScript), you'll need to explicitly call the instance's componentDidMount method call, since the public methods aren't defined until after the component is instantiated.

To test something like this:

Code

public componentDidMount() {  if (this.props.initOpen) {    this.methodName();  }}public methodName = () => {  // some code here}

Test

it('should call methodName during componentDidMount', () => {  const wrapper = mount(<MyComponent {...props} />);  const instance = wrapper.instance();  jest.spyOn(instance, 'methodName')  expect(instance.methodName).toHaveBeenCalled();});