Question

I got this example demonstrating AJAX from Stoyan Stefanovs Object Oriented JavaScript at page 275. In this example, he is requesting three different files. I have a couple questions if any of you can help.

1. What is xhr.send('') doing? Why do we need it? I thought GET in the line before was establishign contact with the server, so why this send?
   (The other questions relate to the closure that I don`t fully understand…)

2. what exactly is getting passed as a paramater to function(myxhr)?

3. regarding the anonymous function, which has (xhr) passed as a parameter, can you explain at what point in the program xhr gets passed to the anonymous function? For example, is it after xhr.open has taken place?

4. why is function(myxhr) necessary? If it`s to create closure, why is closure necessary here?

5. is parameter (xhr) of the anonymous function getting passed as parameter myxhr in function(myxhr) once the anonymous function is called?

6. if 5 is true–that xhr is passed as a parameter to function(myxhr)–why is it necessary to change the parameter name from xhr to myxhr?

Sample code:

function request(url, callback){
  var xhr = new XMLHttpRequest();
  xhr.onreadystatechange = (function(myxhr){
    return function() {
      callback(myxhr);
    }
  })(xhr);
  xhr.open('GET', url, true);
  xhr.send('');
}

request (
  'http://www.phpied.com/files/jsoop/content.txt',
  function (o){
    document.getElementById('text').innerHTML = o.responseText;
  }
);

request(
  'http://www.phpied.com/files/jsoop/content.html',
  function(o) {
    document.getElementById('html').innerHTML = o.responseText;
  }
);

request(
  'http://www.phpied.com/files/jsoop/content.xml',
  function(o){
    document.getElementById('xml').innerHTML =
    o.responseXML.getElementsByTagName('root')[0].firstChild.nodeValue;
  }
);

Answer 1

What is xhr.send(”) doing? Why do we
need it? I thought GET in the line
before was establishign contact with
the server, so why this send? (The
other questions relate to the closure
that I don`t fully understand…)

Open just sets up the request. Send actually sends it. When you make a POST request you need to pass the to send.

what exactly is getting passed as a
paramater to function(myxhr)?

The content of the variable xhr.

regarding the anonymous function,
which has (xhr) passed as a parameter,
can you explain at what point in the
program xhr gets passed to the
anonymous function? For example, is it
after xhr.open has taken place?

As soon as the function is called (which is immediately as the definition is followed by ()).

why is function(myxhr) necessary? If
it`s to create closure, why is closure
necessary here?

It isn’t. Even if the xhr variable wasn’t locally scoped to the request function (which it is) then it could be accessed via this.

is parameter (xhr) of the anonymous
function getting passed as parameter
myxhr in function(myxhr) once the
anonymous function is called?

Yes.

if 5 is true–that xhr is passed as a
parameter to function(myxhr)–why is
it necessary to change the parameter
name from xhr to myxhr?

It isn’t. You can reuse the variable name at different scopes. It makes it less confusing though.