How to test asnyc code with Jest

I am writing react with this starter created by facebook, and I'm a newbie about testing.

Now I have a component about image, it has a function to check Image size:

import React, { Component } from 'react';


class ImagePart extends Component {
    .....
    checkSize(src, width, height){
        let imageObj = new Image();
        imageObj.onload = (evt) => {
            return (evt.target.width >= width && evt.target.height >= height)
            ? true : false;
        }
        imageObj.src = src; 
    }
    .....
}

And test snippet:

import React from 'react';
import ReactDOM from 'react-dom';
import ImagePart from './ImagePart';



it('checking image size without error', () => {
    const image = new ImagePart();
    const img300_300 = 'https://someImage.png';
    expect(image.checkSize(img300_300,200,200)).toEqual(true);
    expect(image.checkSize(img300_300,300,300)).toEqual(true);
    expect(image.checkSize(img300_300,300,200)).toEqual(false);
    expect(image.checkSize(img300_300,200,300)).toEqual(false);
    expect(image.checkSize(img300_300,400,400)).toEqual(false);
});

After Running the test, I got this error :

Expected value to equal: true Received: undefined

I knew the problem is 'onload' is a callback after it got the image.

The question is:
How can I test this without using Promise like this

Do I misunderstand something? Or all I have to do is using promise?

thank you.