function pointers vs direct calling in javascript

[Sampat]

Hi,
I wanted to know the performance of calling a function pointer v/s a
normal function call in javascript in a scenario where there are
multiple calls in the js to the same function. Please share your
thoughts if someone has worked on this.

Thanks,
Sampat.

[Randy Webb]

Sampat said the following on 11/20/2007 6:24 PM:

Hi,
I wanted to know the performance of calling a function pointer v/s a
normal function call in javascript in a scenario where there are
multiple calls in the js to the same function. Please share your
thoughts if someone has worked on this.

You don't need testing to figure it out, although testing will prove it.
The function pointer is *always* going to be slower. It has to be slower
since it it doing more than a plain function call. You are telling the
browser "Do this, then do that" versus "do that" and the first will
*always* be slower.

--
Randy
Chance Favors The Prepared Mind
comp.lang.javas cript FAQ - http://jibbering.com/faq/index.html
Javascript Best Practices - http://www.JavascriptToolbox.com/bestpractices/

[RobG]

On Nov 21, 9:24 am, Sampat <sampatdi...@gm ail.comwrote:

Hi,
I wanted to know the performance of calling a function pointer v/s a
normal function call in javascript in a scenario where there are
multiple calls in the js to the same function. Please share your
thoughts if someone has worked on this.

I don't know what you mean by "function pointer", there is no such
term in javascript. If you mean is it better to assign a reference to
a function rather than use a long lookup chain every time, then the
answer is yes.

e.g. If you need to call the following method a number of times:

baseObj.childOb j.method();
then you can assign a reference to a local variable:

var x = baseObj.childOb j.method;
x();

That should be faster, but probably only if you have to call the
method more than twice within the same execution context. You will
likely only be able to measure the difference if you call the function
10,000 times or so, and users likely won't notice a difference until
you call it 100,000 times (or so).

However, if you mean by "function pointer" that you wrap a method in a
calling function like:

var x = function(){
baseObj.childOb j.method();
}
then see Randy's answer.
--
Rob

[Darko]

On Nov 21, 1:35 am, Randy Webb <HikksNotAtH... @aol.comwrote:

Sampat said the following on 11/20/2007 6:24 PM:

Hi,
I wanted to know the performance of calling a function pointer v/s a
normal function call in javascript in a scenario where there are
multiple calls in the js to the same function. Please share your
thoughts if someone has worked on this.

You don't need testing to figure it out, although testing will prove it.
The function pointer is *always* going to be slower. It has to be slower
since it it doing more than a plain function call. You are telling the
browser "Do this, then do that" versus "do that" and the first will
*always* be slower.

I don't think I understand that. What's "this", and what is "that"?

Darko

[Peter Michaux]

On Nov 21, 9:24 am, Sampat <sampatdi...@gm ail.comwrote:

>
Hi,
I wanted to know the performance of calling a function pointer v/s a
normal function call in javascript in a scenario where there are
multiple calls in the js to the same function. Please share your
thoughts if someone has worked on this.

My guess is that you think the second version below is using a
"function pointer". Both examples evaluate in about the same time in
my Firefox and I'm glad they do because I would be very confused if
they didn't!

----------------------------------------------------------------

function foo() {};

var tic = (new Date()).getTime ();

for (var i=0; i<1e6; i++) {
foo();
}

alert((new Date()).getTime () - tic);

----------------------------------------------------------------

var foo = function() {};

var tic = (new Date()).getTime ();

for (var i=0; i<1e6; i++) {
foo();
}

alert((new Date()).getTime () - tic);

----------------------------------------------------------------

In JavaScript functions are always first class data types so in the
above two examples the identifier "foo" points/refers to the function
object that is defined. The differences in the two examples are the
syntax and also when the function objects are ready to be called when
the JavaScript is first parsed and evaluated.

Peter

[Randy Webb]

RobG said the following on 11/20/2007 9:33 PM:

On Nov 21, 9:24 am, Sampat <sampatdi...@gm ail.comwrote:

>Hi,
I wanted to know the performance of calling a function pointer v/s a
normal function call in javascript in a scenario where there are
multiple calls in the js to the same function. Please share your
thoughts if someone has worked on this.

I don't know what you mean by "function pointer",

If you look at the " llegal operation on WrappedNative prototype object"
thread, you can get a better idea of what he is referring to. He is
referring to a wrapper function.

--
Randy
Chance Favors The Prepared Mind
comp.lang.javas cript FAQ - http://jibbering.com/faq/index.html
Javascript Best Practices - http://www.JavascriptToolbox.com/bestpractices/

[getsanjay.sharma]

On Nov 21, 8:26 am, Randy Webb <HikksNotAtH... @aol.comwrote:

var start2=new Date();
for (var i2=0;i2<100000; i2++){
var tempVar2 = gEBI('myDiv')
var tempVar2 = null;}

Hello Randy.

A quick question, why the 'var' keyword two times? Why not just
'tempVar2 = null'? Is this so that it doesn't go out to search the
scope tree to look for 'tempVar2' or for some other reason? An
detailed explanation with any relevant links would be greatly
appreciated.

Thanks and regards,
/~STS

[getsanjay.sharma]

On Nov 21, 1:13 pm, Randy Webb <HikksNotAtH... @aol.comwrote:

Told ya you wouldn't believe me :)

I believe you. :-)

Is this so that it doesn't go out to search the
scope tree to look for 'tempVar2' or for some other reason?

That's a possibility, although it isn't in this case.

So you mean there are cases where this would be true? Any examples?

An detailed explanation with any relevant links would be greatly
appreciated.

Detailed explanation of the use of the var keyword? I am sure there is
one somewhere but I don't have a link to one. The simplest way to
remember it is that using var on a variable name in a function will
*never* alter a global variable. I always use it unless I explicitly
want to alter a global variable from within a function. The only other
time it can come into play is with inner functions and I never use them.
They give me too big a headache and I have never had a use for them.

Here is what I think of 'var', correct me if I am wrong.

'var' doesn't as such declare a variable. You don't need 'var' keyword
to *declare* variables or put them in the symbol tree in javascript.
It's just an indication to the scripting engine that 'please put this
variable in a scope which is local to this function. So saying:

var a = 10;
var a = 11;

doesn't actually create two variables or declare two variables but
refer to the same variable both the times which is 'a' which was
declared and defined using the statement 'a = 10'. Thus prepending the
variable name with 'var' separates it from the global namespace
(scope) (assuming we are not using 'with' in which case we would have
an additional scope).

Is this good enough?

Thanks and regards,
/~STS

[Randy Webb]

ge************* *@gmail.com said the following on 11/22/2007 1:37 AM:

On Nov 21, 1:13 pm, Randy Webb <HikksNotAtH... @aol.comwrote:

>Told ya you wouldn't believe me :)

I believe you. :-)

>>Is this so that it doesn't go out to search the
scope tree to look for 'tempVar2' or for some other reason?

That's a possibility, although it isn't in this case.

So you mean there are cases where this would be true? Any examples?

A case where I wouldn't want it to go outside looking for it?

var tempVar = new Array();
//lots of entries in tempVar here

function something(){
tempVar = "...."
}

function somethingElse() {
alert('hey, where did my array go?')
}

A case where I would? The altering of a global variable, an array, or
several other things.

>>An detailed explanation with any relevant links would be greatly
appreciated .

Detailed explanation of the use of the var keyword? I am sure there is
one somewhere but I don't have a link to one. The simplest way to
remember it is that using var on a variable name in a function will
*never* alter a global variable. I always use it unless I explicitly
want to alter a global variable from within a function. The only other
time it can come into play is with inner functions and I never use them.
They give me too big a headache and I have never had a use for them.

Here is what I think of 'var', correct me if I am wrong.

'var' doesn't as such declare a variable. You don't need 'var' keyword
to *declare* variables or put them in the symbol tree in javascript.
It's just an indication to the scripting engine that 'please put this
variable in a scope which is local to this function. So saying:

var myArray = new Array();

Give it some thought :)

var a = 10;
var a = 11;

doesn't actually create two variables or declare two variables but
refer to the same variable both the times which is 'a' which was
declared and defined using the statement 'a = 10'. Thus prepending the
variable name with 'var' separates it from the global namespace
(scope) (assuming we are not using 'with' in which case we would have
an additional scope).

Is this good enough?

It isn't that simple though.

var someVar = "global variable";
function someFunction(){
var someVar = "local variable";
someVar = "modified in the function";
alert(window['someVar'])
}

Without testing, what will the alert say, and why? Then test it and see
if you were right without testing it :)
--
Randy
Chance Favors The Prepared Mind
comp.lang.javas cript FAQ - http://jibbering.com/faq/index.html
Javascript Best Practices - http://www.JavascriptToolbox.com/bestpractices/

[Randy Webb]

ge************* *@gmail.com said the following on 11/22/2007 1:37 AM:

(assuming we are not using 'with' in which case we would have an additional scope).

Sidenote: IMNSHO[1], the 'with' operator is for people who don't know
any better. It is worse than eval. There are some weird creepy things
that go on with the 'with' operator.

[1] In My Not So Humble Opinion.
--
Randy
Chance Favors The Prepared Mind
comp.lang.javas cript FAQ - http://jibbering.com/faq/index.html
Javascript Best Practices - http://www.JavascriptToolbox.com/bestpractices/