= function

[maya]

I know how to create functions, call functions, pass vars to functions,
etc.. but what I don't quite get yet is this syntax..

http_request.onreadystatechange = function(){

specifically '= function'...

is this a different way of declaring a function.. if not what
exactly.. I mean this wouldn't make much sense, would it..

function myFunction() {
http_request.onreadystatechange........
}

so if we do

http_request.onreadystatechange = function(){

what are we doing exatly..

am learning AJAX so also need to learn more JavaScript...:)

thank you very much..

[lofty00]

maya wrote:

I know how to create functions, call functions, pass vars to functions,
etc.. but what I don't quite get yet is this syntax..

http_request.onreadystatechange = function(){

specifically '= function'...

What you are doing is creating an unnamed function object, and setting
http_request.onreadystatechange as a pointer to it. This function will
end up as one of the parameters of your http request. The request code
needs to know what to do when the request succeeds because it doesn't
have any built in default behaviour to do this, so it takes a function
as a parameter. You can do stuff like this in your own code. e.g. this
is perfectly valid javascript:

function callbackFunc() {
alert("here we are");
}

function callerFunc(callback) {
// note that callback is a function parameter/
callback();
}

// call the callback function.
// here you are passing callerFunc the name of a function as a
parameter.
callerFunc(callbackFunc);

[Martin Honnen]

maya wrote:

I know how to create functions, call functions, pass vars to functions,
etc.. but what I don't quite get yet is this syntax..

http_request.onreadystatechange = function(){

Functions are first class citizens in JavaScript so the same way you can
write expressions to yield string or number or boolean values you can
write an expression that yields a function. So
function () { }
on the right hand side of the assignment is a function expression that
yields an anonymous function object as its result.
--

Martin Honnen
http://JavaScript.FAQTs.com/

[Elegie]

maya wrote:

I know how to create functions, call functions, pass vars to functions,
etc.. but what I don't quite get yet is this syntax..

http_request.onreadystatechange = function(){
In javascript, functions can be defined in two ways:

[1] function foo() {...}
[2] var foo = function() {...}

The [1] refers to what the ECMAScript specification names a "function
declaration"; it is a function that is parsed by the interpreter even
before trying to execute the source code. When you define a function
this way, you have to give a name to the function.

The [2] refers to a "function expression"; it is parsed when the
interpreter gets to the line, as a normal expression. When you define a
function this way, you do not have to give a name to the function (it
can stay anonymous, and you just hold a reference to it in your variable).

am learning AJAX so also need to learn more JavaScript...:)

The fact that functions are objects in javascript, opens a full range of
subtle possibilities with javascript, yet maybe difficult to grasp fully
for a beginner - prepare the aspirin :)

<URL:http://en.wikipedia.org/wiki/First_class_function>
<URL:http://en.wikipedia.org/wiki/Closure_(computer_science)>

[maya]

Martin Honnen wrote:

maya wrote:

I know how to create functions, call functions, pass vars to
functions, etc.. but what I don't quite get yet is this syntax..

http_request.onreadystatechange = function(){

Functions are first class citizens in JavaScript so the same way you can
write expressions to yield string or number or boolean values you can
write an expression that yields a function. So
function () { }
on the right hand side of the assignment is a function expression that
yields an anonymous function object as its result.

thank you Martin.. so how do you use this "anonymous function object"...
I'm looking at JavaScript Bible; this is entire explanation he gives
of this technique:

One last function creation format is available in NN4+. This advanced
technique, called a lambda expression, provides a shortcut for creating
a reference to an anonymous function (truly anonymous because the
function has no name that you can reference later). The common
application of this technique is to assign function references to event
handlers when the NN event object also must be passed. The following is
an example of how to assign an anonymous function to an onChange event
handler for a form control:

document.forms[0].age.onchange = function(event)
{isNumber(document.forms[0].age)}

I still don't get what he's doing here.. (would this be the same as
declaring

function isNumber() { // etc...}
(maybe passing it some params..)

in other words is this an alternative way of declaring a function.. or
is it something different altogether..

(does he mean like anomymous classes in Java.. I know Java, but find
anonymous classes confusing...)

thank you..

[Martin Honnen]

maya wrote:

so how do you use this "anonymous function object"...

Like any function, in your original example the function is assigned to
the onreadystatechange event handler meaning the XMLHttpRequest object
will call the function whenever the readyState changes.

--

Martin Honnen
http://JavaScript.FAQTs.com/