Question

119

return value calculated from javascript FileReader onload event

rated 0 times [ 121] [ 2] / answers: 1 / hits: 19436 / 9 Years ago, fri, april 24, 2015, 12:00:00

I have this function:

function doStuff(range, file) {

    var fr = new FileReader();

    var hash = '';

    fr.onload = function (e) {

        var out = stuff happens here;

        hash = asmCrypto.SHA256.hex(out);

        return hash;

    };

    fr.readAsArrayBuffer(file);

    return hash;

}

Right now, the function completes before the onload event is finished, so doStuff always returns . I think a callback is what I need, but I'm new to javascript, and I can't wrap my mind around how to implement it in this case.

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claudiofredye

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answered 9 Years ago leifw · Accepted Answer

File reading using File Reader is asynchronous operation. Place your logic inside the onload function of file reader.

function doStuff(range, file) {

    var fr = new FileReader();

    fr.onload = function (e) {

        var out = stuff happens here;

        hash = asmCrypto.SHA256.hex(out);

        /* Place your logic here */

    };

    fr.readAsArrayBuffer(file);

}

You can even pass a callback function that will be executed once the file is read.

function doStuff(range, file, callback) {

    var fr = new FileReader();

    fr.onload = function (e) {

        var out = stuff happens here;

        hash = asmCrypto.SHA256.hex(out);

        /* Assuming callback is function */

        callback(hash);

    };

    fr.readAsArrayBuffer(file);

}