Question

I need to generate a result from 2 XMLHttpRequests.
How can I make the requests concurrently and wait for them to both finish?

I’ve though of something like…

resp1="";
req1.onreadystatechange=function(){if(this.readyState=4)resp1==this.responseText;}
req2.onreadystatechangefunction(){if(this.readyState=4) finish(this.responseText);}
function finish(resp2){
if (resp1=="") setTimeOut(finish(resp2),200);
else {
... both are done...
}

I haven’t tested it yet but I assume it would work.
Is there a better way?
My code needs to be as short and fast as possible.

Answer 1

You don’t need a timer for this.

All you need to do is check in each callback whether the other one finished, and, if so, call finish.

For example:

var resp1 = null, resp2 = null;

req1.onreadystatechange = function() {
    if (this.readyState === 4) {
        resp1 = this.responseText;
        if (resp2 !== null) finish();
    }
};
req2.onreadystatechange = function() {
    if (this.readyState === 4) {
        resp2 = this.responseText;
        if (resp1 !== null) finish();
    }
};