How to upload files in Web Workers when FormData is not defined

I have written the following polyfill to emulate the FormData method in Web Workers. Since Web workers do not support DOM, the new FormData(<HTMLFormElement>); constructor call is not supported either.
File and Blob objects, typed arrays and strings are supported by the polyfill though.

It was originally posted as a part of the answer to Upload a File in a Google Chrome Extension. To see an example of how it can be used, have a look at the other answer.

/* * FormData for XMLHttpRequest 2  -  Polyfill for Web Worker  (c) 2012 Rob W * License: Creative Commons BY - http://creativecommons.org/licenses/by/3.0/ * - append(name, value[, filename]) * - toString: Returns an ArrayBuffer object *  * Specification: http://www.w3.org/TR/XMLHttpRequest/#formdata *                http://www.w3.org/TR/XMLHttpRequest/#the-send-method * The .append() implementation also accepts Uint8Array and ArrayBuffer objects * Web Workers do not natively support FormData: *                http://dev.w3.org/html5/workers/#apis-available-to-workers **/(function() {    // Export variable to the global scope    (this == undefined ? self : this)['FormData'] = FormData;    var ___send$rw = XMLHttpRequest.prototype.send;    XMLHttpRequest.prototype['send'] = function(data) {        if (data instanceof FormData) {            if (!data.__endedMultipart) data.__append('--' + data.boundary + '--\r\n');            data.__endedMultipart = true;            this.setRequestHeader('Content-Type', 'multipart/form-data; boundary=' + data.boundary);            data = new Uint8Array(data.data).buffer;        }        // Invoke original XHR.send        return ___send$rw.call(this, data);    };    function FormData() {        // Force a Constructor        if (!(this instanceof FormData)) return new FormData();        // Generate a random boundary - This must be unique with respect to the form's contents.        this.boundary = '------RWWorkerFormDataBoundary' + Math.random().toString(36);        var internal_data = this.data = [];        /**        * Internal method.        * @param inp String | ArrayBuffer | Uint8Array  Input        */        this.__append = function(inp) {            var i=0, len;            if (typeof inp === 'string') {                for (len=inp.length; i<len; i++)                    internal_data.push(inp.charCodeAt(i) & 0xff);            } else if (inp && inp.byteLength) {/*If ArrayBuffer or typed array */                if (!('byteOffset' in inp))   /* If ArrayBuffer, wrap in view */                    inp = new Uint8Array(inp);                for (len=inp.byteLength; i<len; i++)                    internal_data.push(inp[i] & 0xff);            }        };    }    /**    * @param name     String                                  Key name    * @param value    String|Blob|File|Uint8Array|ArrayBuffer Value    * @param filename String                                  Optional File name (when value is not a string).    **/    FormData.prototype['append'] = function(name, value, filename) {        if (this.__endedMultipart) {            // Truncate the closing boundary            this.data.length -= this.boundary.length + 6;            this.__endedMultipart = false;        }        var valueType = Object.prototype.toString.call(value),            part = '--' + this.boundary + '\r\n' +                 'Content-Disposition: form-data; name="' + name + '"';        if (/^\[object (?:Blob|File)(?:Constructor)?\]$/.test(valueType)) {            return this.append(name,                            new Uint8Array(new FileReaderSync().readAsArrayBuffer(value)),                            filename || value.name);        } else if (/^\[object (?:Uint8Array|ArrayBuffer)(?:Constructor)?\]$/.test(valueType)) {            part += '; filename="'+ (filename || 'blob').replace(/"/g,'%22') +'"\r\n';            part += 'Content-Type: application/octet-stream\r\n\r\n';            this.__append(part);            this.__append(value);            part = '\r\n';        } else {            part += '\r\n\r\n' + value + '\r\n';        }        this.__append(part);    };})();