stubbing a function using jest

Question

is there a way to stub a function using jest API? I\'m used to working with sinon stub, where I can write unit-tests with stubs for any function call coming out of my tested un

Answer 1

Jest provides jest.fn(), which has some basic mocking and stubbing functionality.

If you're experienced and comfortable with sinon you could still create Jest-based tests which use sinon test doubles. However you'll lose the convenience of built in Jest matchers such as expect(myStubFunction).toHaveBeenCalled().

Answer 2

With jest you should use jest.spyOn:

jest
  .spyOn(jQuery, "ajax")
  .mockImplementation(({ success }) => success([ 1, 2, 3 ]));

Full example:

const spy = jest.fn();
const payload = [1, 2, 3];

jest
  .spyOn(jQuery, "ajax")
  .mockImplementation(({ success }) => success(payload));

jQuery.ajax({
  url: "https://example.api",
  success: data => spy(data)
});

expect(spy).toHaveBeenCalledTimes(1);
expect(spy).toHaveBeenCalledWith(payload);

You can try live example on codesandbox: https://codesandbox.io/s/018x609krw?expanddevtools=1&module=%2Findex.test.js&view=editor

Answer 3

I was able to sub out jquery entirely by using mockReturnValue and jquery's $.Deferred. This allowed me to manually resolve my ajax calls and then the rest of the function would continue (and any chaining of .done() or .success() etc would execute.

Example:

const deferred = new $.Deferred();
$.ajax = jest.fn().mockReturnValue(deferred);

myClass.executeAjaxFunction();

const return_val = 7;

deferred.resolve(return_val)

Then if I have a function like

$.ajax({
    type: 'GET',
    url: '/myurl'
}).done((val) => {
    window.property = val;
});

The following test will pass

it('should set my property correctly', () => {
    expect(window.property).toBe(7);
});

Of course- you can skip the deferred part of this answer if you are trying to stub a non-jquery function. I came across this question that dealt with ajax and came up with this solution as a way to test a function that executes actions after an ajax call is complete using Jest.