Chrome extension: How to save a file on disk

I had long been wishing to make a chrome extension for myself to batch download images. Yet every time I got frustrated because the only seemingly applicable option is NPAPI, which both chrome and firefox seem to have not desire in supporting any longer.

I suggest those who still wanted to implement 'save-file-on-disk' functionality to have a look at this Stackoverflow post, the comment below this post help me a lot.

Now since chrome 31+, the chrome.downloads API became stable. We can use it to programmatically download file. If the user didn't set the ask me before every download advance option in chrome setting, we can save file without prompting user to confirm!

Here is what I use (at extension's background page):

    // remember to add "permissions": ["downloads"] to manifest.json
    // this snippet is inside a onMessage() listener function
    var imgurl = "https://www.google.com.hk/images/srpr/logo11w.png";
    chrome.downloads.download({url:imgurl},function(downloadId){
        console.log("download begin, the downId is:" + downloadId);
    });

Though it's a pity that chrome still doesn't provide an Event when the download completes.chrome.downloads.download's callback function is called when the download begin successfully (not on completed)

The Official documentation about chrome.downloadsis here.

It's not my original idea about the solution, but I posted here hoping that it may be of some use to someone.

There's no way that I know of to silently save files to the user's drive, which is what it seems like you're hoping to do. I think you can ASK for files to be saved one at a time (prompting the user each time) using something like:

function saveAsMe (filename)
{
document.execCommand('SaveAs',null,filename)
}

If you wanted to only prompt the user once, you could grab all the images silently, zip them up in a bundle, then have the user download that. This might mean doing XmlHttpRequest on all the files, zipping them in Javascript, UPLOADING them to a staging area, and then asking the user if they would like to download the zip file. Sounds absurd, I know.

There are local storage options in the browser, but they are only for the developer's use, within the sandbox, as far as I know. (e.g. Gmail offline caching.) See recent announcements from Google like this one.

You can use HTML5 FileSystem features to write to disk using the Download API. That is the only way to download files to disk and it is limited.

You could take a look at NPAPI plugin. Another way to do what you need is simply send a request to an external website via XHR POST and then another GET request to retrieve the file back which will appear as a save file dialog.

For example, for my browser extension My Hangouts I created a utility to download a photo from HTML5 Canvas directly to disk. You can take a look at the code here capture_gallery_downloader.js the code that does that is:

var url = window.webkitURL || window.URL || window.mozURL || window.msURL;
var a = document.createElement('a');
a.download = 'MyHangouts-MomentCapture.jpg';
a.href = url.createObjectURL(dataURIToBlob(data.active, 'jpg'));
a.textContent = 'Click here to download!';
a.dataset.downloadurl = ['jpg', a.download, a.href].join(':');

If you would like the implementation of converting a URI to a Blob in HTML5 here is how I did it:

/**
 * Converts the Data Image URI to a Blob.
 *
 * @param {string} dataURI base64 data image URI.
 * @param {string} mimetype the image mimetype.
 */
var dataURIToBlob = function(dataURI, mimetype) {
  var BASE64_MARKER = ';base64,';
  var base64Index = dataURI.indexOf(BASE64_MARKER) + BASE64_MARKER.length;
  var base64 = dataURI.substring(base64Index);
  var raw = window.atob(base64);
  var rawLength = raw.length;
  var uInt8Array = new Uint8Array(rawLength);

  for (var i = 0; i < rawLength; ++i) {
    uInt8Array[i] = raw.charCodeAt(i);
  }

  var bb = new this.BlobBuilder();
  bb.append(uInt8Array.buffer);
  return bb.getBlob(mimetype);
};

Then after the user clicks on the download button, it will use the "download" HTML5 File API to download the blob URI into a file.