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, callback) => {

  const MAX_WIDTH = avatarImage.maxWidth;

  const MAX_HEIGHT = avatarImage.maxHeight;

  const img = document.createElement('img');

  img.src = window.URL.createObjectURL(file);

  const canvas = document.createElement('canvas');

  const ctx = canvas.getContext('2d');



  img.onload = () => {

    const sizes = internalResizeImage(img, MAX_WIDTH, MAX_HEIGHT);



    canvas.width = sizes.width;

    canvas.height = sizes.height;

    ctx.drawImage(img, 0, 0, sizes.width, sizes.height);

    return callback(dataURItoFile(canvas.toDataURL(), fileName));

  };

};

I called like this:

resizeImage(acceptedFiles[0], this.props.user.id, (res) => {

  //dostuff

});

In my test I'm mocking it like this:

let mockResizeImage = jest.fn();



jest.mock('../../utilities/imageUtils', () => ({

  resizeImage: () => mockResizeImage

}));

I want mockResizeImage to be a callback and then in my test change the returning values:

it('should call handleDrop and accept files', () => {

    //mockResizeImage.mockReturnValue('something');



    const instance = shallow(mockComponent()).instance();

    const acceptFilesMock = ['test'];



    instance.handleDrop(acceptFilesMock);



    expect(clickSpy).toHaveBeenCalledTimes(1);

  });

If it were a promise, all good, but it's a callback and I don't know what I'm doing wrong.

Thanks.