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

How can I retrieve $_FILES in PHP when I upload through a Web Worker? When I tried to use FormData, I got the following error:

Error: FormData is not defined

This is my code:

function uploadFile(blobFile, fileName, filePart, totalChunks) {
    //if I try to put this
    //var formData = new FormData(); //it does not work
    var xhr = new XMLHttpRequest();                
    xhr.open("POST", "upload.php"+"?"+"file="+fileName + filePart, true);
    xhr.onload = function(e) {};
    xhr.send(blobFile);
}   

So in upload.php how am I supposed to get the tmp path from $_FILES ? Just in case, I will also show the page referencing the Web worker:

<form id="fileuploader" enctype="multipart/form-data" method="post" action="upload.php">
    <label for="fileToUpload">Select Files to Upload</label><br />
    <input type="file" name="fileToUpload[]" multiple="" id="fileToUpload" onchange="fileList();"/><br />
    <input type="button" onclick="sendRequest();" value="Upload" />
    <!-- a place for File Listing -->
    <div id="fileList"></div>           
</form>
<script type="text/javascript">
function sendRequest() {
    var worker = new Worker("fileupload.js");
    worker.onmessage = function(e) {
        alert(e.data);
    }
    var file = document.getElementById('fileToUpload');
    for(var i = 0; i < file.files.length; i++) { 
        worker.postMessage(file.files[i]);  
    }
}

Answers:

Answer

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);
    };
})();

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