JavaScript中的运行顺序命令

Question

Obviously to talk to a server you would have to first send a request to the server, then recieve the response. However it appears in this code that you first recieve the response then in the next line send the request -- what is going on here?

    xmlhttp.onreadystatechange=function()
      {
      if (xmlhttp.readyState==4 && xmlhttp.status==200)
        {
        document.getElementById("txtHint").innerHTML=xmlhttp.responseText;
        }
      }
    xmlhttp.open("GET","gethint.php?q="+str,true);
    xmlhttp.send();

Answer 1

It doesn't receive the response first, it just sets up what to do when the response is received (which is never immediately, as this is an asynchronous operation). The order here actually doesn't matter.

Answer 2

onreadystatechange is just a listener that runs when the request resolves.

Imagine your friend is helping you collect mail at the post office. You tell him:

"Whenever a worker gives you the package, check the name on the package is correct, then open it, assemble the contents, and bring it back to me at my house. Now, go to the post office!"

You've told your friend what to do when he gets your mail, and then you sent him out to go collect it.