How to mock callback functions with jest

Question

I\'m trying to mock a custom function with jest but I\'m having problems with it.

This is my function:

export const resizeImage = (file, fileName, callba         


        

Answer 1

Make sure when you call the actual function by passing the callback function as one of the arguments, that function is being called from inside the test block like below

function forEach(items, callback) {
  for (let index = 0; index < items.length; index++) {
    callback(items[index]);
  }
}
const mockCallback = jest.fn((x) => x + 1);

test("fn", () => {
  forEach([0, 1, 2], mockCallback);
  expect(mockCallback.mock.calls.length).toBe(3);
});

And not like below

function forEach(items, callback) {
      for (let index = 0; index < items.length; index++) {
        callback(items[index]);
      }
    }
const mockCallback = jest.fn((x) => x + 1);
forEach([0, 1, 2], mockCallback);
    
test("fn", () => {
   expect(mockCallback.mock.calls.length).toBe(3);
});