Question

I am working with onkeydown events in order to get the written value (in a textarea), process it on a PHP file (passing it by Ajax post) and showing the result in an external div…
The problem is that each time I press the key, I can’t continue writing till Ajax does its task. How can I make it possible to keep writing even if the text hasn’t been processed yet?

—EDIT—

function fromWCtoHTML(source){
 var act = new Date();
 http[act] = createRequestObject();
 http[act].open('post', '/php/fromWCtoHTML.php',false);
 http[act].setRequestHeader('Content-Type','application/x-www-form-urlencoded',false);
 http[act].send("source=" + source);
 document.getElementById('AJAXWCtoHTML').innerHTML=http[act].responseText;
}

Answer 1

http[act].open('post', '/php/fromWCtoHTML.php',false);

The 3rd parameter[false] is causing your request to be synchronous.
Change it to true or do not include it.

EDIT Response for the comment stating that http[act].responseText is blank.

Add an onreadystate event handler. Try the following code

function fromWCtoHTML(source){
 var act = new Date();
 http[act] = createRequestObject();
 http[act].open('post', '/php/fromWCtoHTML.php');
 http[act].setRequestHeader('Content-Type','application/x-www-form-urlencoded',false);
 http[act].send("source=" + source);
  http[act].onreadystatechange=function(){
 if (http[act].readyState==4 && xmlhttp.status==200)
    {
    document.getElementById('AJAXWCtoHTML').innerHTML=http[act].responseText;
    }
}