How to add header data in XMLHttpRequest when using formdata?

Your error

InvalidStateError: An attempt was made to use an object that is not, or is no longer, usable

appears because you must call setRequestHeader after calling open. Simply move your setRequestHeader line below your open line (but before send):

xmlhttp.open("POST", url);xmlhttp.setRequestHeader("x-filename", photoId);xmlhttp.send(formData);

Use: xmlhttp.setRequestHeader(key, value);

Check to see if the key-value pair is actually showing up in the request:

In Chrome, found somewhere like: F12: Developer Tools > Network Tab > Whatever request you have sent > "view source" under Response Headers

Depending on your testing workflow, if whatever pair you added isn't there, you may just need to clear your browser cache. To verify that your browser is using your most up-to-date code, you can check the page's sources, in Chrome this is found somewhere like: F12: Developer Tools > Sources Tab > YourJavascriptSrc.js and check your code.

But as other answers have said:

xhttp.setRequestHeader(key, value);

should add a key-value pair to your request header, just make sure to place it after your open() and before your send()