POSTing Multipart/Form-Data from an XMLHttpRequest

I’m working on an add-on where I have a need to POST JSON data to the server. Unfortunately that JSON contains ampersands, so in order to use POST, I would have to encode the data in some way that the server could decode. This seemed like extra unnecessary work to me. I decided to use the “multipart/form-data” header which would allow me to send the data unmodified. Unfortunately, documentation on this was lacking. So this post is just to put up sample code on how to do this in case someone else needs it. In this case, I am simply passing data=foo where foo is the JSON.

var xhr = Components.classes[";1"].createInstance(Components.interfaces.nsIXMLHttpRequest);"POST", url, true);

var boundary = '---------------------------';
boundary += Math.floor(Math.random()*32768);
boundary += Math.floor(Math.random()*32768);
boundary += Math.floor(Math.random()*32768);
xhr.setRequestHeader("Content-Type", 'multipart/form-data; boundary=' + boundary);
var body = '';
body += '--' + boundary + '\r\n' + 'Content-Disposition: form-data; name="';
body += "data";
body += '"\r\n\r\n';
body += JSON.stringify(JAVASCRIPT OBJECT);
body += '\r\n'
body += '--' + boundary + '--';
xhr.setRequestHeader('Content-length', body.length);
xhr.onload = function() {

There were no server changes required at all. The PHP handled the data the same way it would have if it was an “application/x-www-form-urlencoded” POST.

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