webamp/js/my-file.js

// Custom object representing a file
// `File` is already a builtin, so we use `MyFile`
define([], function(){
return function() {
    this.reader = new FileReader();
    this.url = null;
    this.fileReference = null;
    this.setUrl = function(url){
        this.url = url;
    };
    this.setFileReference = function(fileReference){
        this.fileReference = fileReference;
    };
    this.processBuffer = function(bufferHandler) {
        if(this.url) {
            var oReq = new XMLHttpRequest();
            oReq.open("GET", this.url, true);
            oReq.responseType = "arraybuffer";

            oReq.onload = function (oEvent) {
                var arrayBuffer = oReq.response; // Note: not oReq.responseText
                bufferHandler(arrayBuffer);
            };

            oReq.send(null);
            return;

        } else if(this.fileReference) {
            this.reader.onload = function (e) {
                var arrayBuffer = e.target.result;
                bufferHandler(arrayBuffer);
            };
            this.reader.onerror = function (e) {
                console.error(e);
            };

            this.reader.readAsArrayBuffer(this.fileReference);
            return;
        }

        console.error('Tried to process an unpopulated file object');
        return false;
    };
};
});