How can I return a value from two asynchronous method?

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I am trying to create an image compressor without any third-party library.

Here is my code(it will finally returns the blob of compressed image):

 async function CompressImage(File, Quality, FileType) {
        var canvas = document.createElement('canvas');
        var ctx = canvas.getContext('2d');
        var img = new Image;
        img.onload = function () {
            canvas.width = img.width;
            canvas.height = img.height;
            ctx.drawImage(img, 0, 0, img.width, img.height);                        
                canvas.toBlob(function (blob) {                    
                    return blob;
                }, FileType, Quality);            
            }
        }
        img.src = URL.createObjectURL(File);
    }

It seems there is nothing wrong with my code while always returns an undefined value.

I found out maybe it is the asynchronous problem. The img.onload and the canvas.toBlob are asynchronous methods.

I think the promise may help this while I don’t know how to order these methods run one by one and finally returns a value.

How can I solve this? Thank you.

174 thoughts on “How can I return a value from two asynchronous method?”

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