Upload a File in a Google Chrome Extension

I’ve recently developed a Chrome extension which retrieves content from a page, and sends it to the server.

The following approach was used:

File downloads: Get the src property of an <img> element, for example.
Fetch the file from the Cache – use XMLHttpRequest from the background page.
Use a Web Worker in the background page to handle the upload.

Side note, to take the checksum of the image, Crypto-JS: MD5 can be used. Example (where xhr is the XMLHttpRequest object with responseType set to arraybuffer, see Worker demo):

var md5sum = Crypto.MD5( new Uint8Array(xhr.response) );

Full example

Content script

// Example: Grab the first <img> from the document if it exists.
var img = document.images[0];
if (img) {
    // Send the target of the image:
    chrome.runtime.sendMessage({method: 'postUrl', url: img.src});
}

Background script (with Worker)

chrome.runtime.onMessage.addListener(function(request) {
    if (request.method == 'postUrl') {
        var worker = new Worker('worker.js');
        worker.postMessage(request.url);
    }
});

Web Worker

// Define the FormData object for the Web worker:
importScripts('xhr2-FormData.js')

// Note: In a Web worker, the global object is called "self" instead of "window"
self.onmessage = function(event) {
    var resourceUrl = event.data;   // From the background page
    var xhr = new XMLHttpRequest();
    xhr.open('GET', resourceUrl, true);

    // Response type arraybuffer - XMLHttpRequest 2
    xhr.responseType="arraybuffer";
    xhr.onload = function(e) {
        if (xhr.status == 200) {
            nextStep(xhr.response);
        }
    };
    xhr.send();
};
function nextStep(arrayBuffer) {
    var xhr = new XMLHttpRequest();
    // Using FormData polyfill for Web workers!
    var fd = new FormData();
    fd.append('server-method', 'upload');

    // The native FormData.append method ONLY takes Blobs, Files or strings
    // The FormData for Web workers polyfill can also deal with array buffers
    fd.append('file', arrayBuffer);

    xhr.open('POST', 'http://YOUR.DOMAIN.HERE/posturl.php', true);

    // Transmit the form to the server
    xhr.send(fd);
};

`FormData` for Web workers POLYFILL

Web workers do not natively support the FormData object, used to transmit multipart/form-data forms. That’s why I’ve written a polyfill for it. This code has to be included in the Web worker, using importScripts('xhr2-FormData.js').

The source code of the polyfill is available at https://gist.github.com/Rob–W/8b5adedd84c0d36aba64

Manifest file:

{
  "name": "Rob W - Demo: Scraping images and posting data",
  "version": "1.0",
  "manifest_version": 2,
  "content_scripts": [
    {
      "matches": ["http://*/*", "https://*/*"],
      "js": ["contentscript.js"]
    }
   ],
   "background": {
       "scripts": ["background.js"]
   },
   "permissions": ["http://*/*", "https://*/*"]
}

Relevant documentation

Message passing ^{Google Chrome Extensions}
chrome.runtime.onMessage ^{Google Chrome Extensions}
XMLHttpRequest Level 2 ^{W3c specification}
FormData (XHR2) ^MDN
ArrayBuffer responses (XHR2) ^{HTML5 rocks} (note: arraybuffer responses are deprecated in favor of typed arrays, the polyfill has been updated to reflect this change)