Question

xmlhttp.onreadystatechange=function() {

So this says onreadystatechange, invoke function(). Can I put parameters in function()? Second question, what does it mean when someone writes, xmlhttp.onreadystatechange=statechanged? Does that mean it will always be true or something?

Answer 1

1. You can’t use a parameter since onreadystatechange has no parameters to give you. What parameter are you expecting? It is simply a hook for handling the response. What you do have available is the xmlhttp.readyState which tells you if the response is ready or not, xmlhttp.status – the http status code (i.e. 200), and xmlhttp.responseText – the response itself.

2. No – that means you are assigning a variable reference (a function) to onreadystatechange.

I would strongly suggest using a JS framework (such as jQuery) to do AJAX calls – it will abstract away the low-level details you are asking about. If you must use native JS AJAX calls – read this tutorial.