JavaScript set value to variable defined in parent function

Question

i want to return imageSize but it is returning undefined. i am confused i have tried many things. in the alert i am getting size.

getImageSize : function(file) {
        var reader = new FileReader();
        var image  = new Image();
        var imageSize;
        reader.readAsDataURL(file);  
        reader.onload = function(_file) {
            image.src    = _file.target.result;
            image.onload = function() {
                imageSize = ~~(file.size/1024) +'KB';
                alert(imageSize);
            };     
        };
        return imageSize;
    }

This is because imageSize is still undefined by the time the return line is reached. The value of imageSize is updated only when the two onload events are fired. — tewathia
– tewathia, Commented Nov 20, 2013 at 6:31
is there any way to re-write this function so that i can return imageSize? — vis
– vis, Commented Nov 20, 2013 at 6:33

Ted Hopp · Accepted Answer · 2013-11-20 06:33:46Z

1

Since you are loading the image asynchronously, you cannot return the size directly. The best you can do is pass in a call-back function to be called when the size is available:

getImageSize : function(file, callback) {
    var reader = new FileReader();
    var image  = new Image();
    var imageSize;
    reader.readAsDataURL(file);  
    reader.onload = function(_file) {
        image.src    = _file.target.result;
        image.onload = function() {
            imageSize = ~~(file.size/1024) +'KB';
            callback(imageSize);
        };     
    };
}

answered Nov 20, 2013 at 6:33

Ted Hopp

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