Javascript/HTML5 file API reading sequential files into multipart form data

I'm using the HTML5 File API to assemble multipart form data for submission by XHR to a web service. I have the whole thing working in FF, which has a nice convenient getAsBinary() method included in their implementation of the file API. This was a pretty sweet deal. It basically went:

var const;  // constructor
const += headers;
const += field_data;

for(var i = 0; i < files.length; i++)
{
   const += files[i].getAsBinary();
}

sendData(const);

Worked like a charm.

To get it working in Chrome, though, I have to make a FileReader object, which handles a bit differently. I essentially have to go:

var const;  // constructor
const += headers;
const += field_data;

var reader = new FileReader();

for(var i = 0; i < files.length; i++)
{
    reader.onload = (function(file)
    {
       const += file.target.result;   // const is not in scope in this anonymous function!
    }
    reader.readAsBinaryString(files[i]);
}

sendData(const);

Which doesn't work, for two main reasons. Firstly, the read happens asynchronously, so by the time it gets to the sendData() function, the file data isn't written to the const variable. Secondly, the const variable is out of scope inside the reader.onload handler. However I re-jig the code, I seem to come across one of these obstacles, and I'm struggling to come up with a graceful way of handling it.

Any suggestions?

What you're going to have to do is have the reader "load" handlers all check to see whether they're the last one to run. When that happens, then that handler can call "sendData()".

var const;  // constructor
const += headers;
const += field_data;

var reader;
var finished = 0;
for(var i = 0; i < files.length; i++)
{
    reader = new FileReader();
    reader.onload = function(file)
    {
       const += file.target.result;
       if (++finished === files.length)
         sendData(const);
    };
    reader.readAsBinaryString(files[i]);
}

(I don't fully understand the details of how that accumulated "const" thing will correctly turn into a multipart MIME blob, but I presume that you do :-) Also, and this is probably important: I think you probably need to make a new "FileReader" instance for each file. I coded this that way (actually I just edited it) but that may be incorrect, as I'm not that familiar with the API and its semantics.

Have you considered xhr.send(FormData)? See my response here: upload to php $_FILE from chrome extension

You can append files to a FormData object and send that via xhr. The browser constructs the multi-part request for you. I believe this is available in FF4 and has been in Chrome for some time.