File API not working with Blob in Google Chrome Extension

Question

I'm writing an extension for Google Chrome where I need to download the images of a page and save them on disk on temporary files. To do this, I make an XMLHttpRequest and use it to download the image. In the callback function, when I have the image in request.response, I request a temporary file from Windows using window.webkitRequestFileSystem(window.TEMPORARY, 1024 * 1024 * 20, writeImageToFile, errorHandler). In the callback function writeImageToFile, I create the FileWriter object and write the blob into it. The file is getting created but I'm not able to find out where it is being created. I need the windows path of the file for the extension. Please help!

Answer 1

For your purpose, you do not need a virtual file system. Use the webkitURL.createObjectURL method to create a URL from a blob:

window.webkitURL.createObjectURL(blob);

Since Chrome 19, you can directly get a Blob object using responseType:

var xhr = new XMLHttpRequest();
xhr.responseType = 'blob';
xhr.onload = function() {
    // xhr.response is a Blob
    var url = webkitURL.createObjectURL(xhr.response);
    console.log('URL: ', url);
};
xhr.open('GET', 'http://example.com/favicon.ico');
xhr.send();

Since Chrome 10, you can get an ArrayBuffer response, and convert it to a blob (demo):

var xhr = new XMLHttpRequest();
xhr.responseType = 'arraybuffer';
xhr.onload = function() {
    // Create blob from arraybuffer
    var bb = new (window.BlobBuilder || window.WebKitBlobBuilder)();
    bb.append(xhr.response);
    var url = webkitURL.createObjectURL(bb.getBlob());
    console.log('URL: ', url);
};
xhr.open('GET', 'http://example.com/favicon.ico');
xhr.send();

(Use window.URL and MozBlobBuilder if you want to use the same code for Firefox).