javascript - HTML5 FileReader how to return result?

Question

I use JS FileReader. I want to get result after file reading operation and work with this data. FileReader is asynchronous and I don't know when result is ready. How to get it done right?

$(document).ready(function(){
    $('#file_input').on('change', function(e){
        var res = readFile(this.files[0]);

        //my some manipulate with res

        $('#output_field').text(res);
    });
});

function readFile(file){
    var reader = new FileReader(),
        result = 'empty';

    reader.onload = function(e)
    {
        result = e.target.result;
    };

    reader.readAsText(file);

    //waiting until result is empty?

    return result;
}

http://jsfiddle.net/ub22m/4/

It's show "empty".

How to wait until "result" is empty? Another way?

Answer 1

Reading happens asynchronously. You need to provide a custom onload callback that defines what should happen when the read completes:

$(document).ready(function(){
    $('#file_input').on('change', function(e){
        readFile(this.files[0], function(e) {
            // use result in callback...
            $('#output_field').text(e.target.result);
        });
    });
});

function readFile(file, onLoadCallback){
    var reader = new FileReader();
    reader.onload = onLoadCallback;
    reader.readAsText(file);
}

(See the Fiddle.)

Note that readFile does not return a value. ?Instead, it accepts a callback function, which will fire whenever the read is done. The $('#output_field').text operation is moved into the callback function. This ensures that that operation will not run until the reader's onload callback fires, when e.target.result will be filled.

Programming with callbacks is a bit difficult to get right at first, but it is absolutely necessary for implementing advanced functionality.