Why user private function?

Question

So the have seem something like this today:

$$('input').each(function() {
 // do something
});

What is the difference between that and this:

$$('input').each( // do something );

The private function on the first example is within that for loop scope, so it is not reference-able anywhere else, so what would the benefit of having it?

... not using a function literal would throw a syntax error, because arbitrary JavaScript can't be embedded in parentheses? — Ry-, Mar 23 '12 at 14:51
You _could_ define an external function and pass it in to each like so: `function foo() { alert('foo!') }; $('.input').each(foo);` OR you could make an anonymous function and pass it in like you did in your first example: `$('.input').each( function() { alert('foo!') } );` — rcplusplus, Mar 23 '12 at 14:54
@minitech, so what you are saying is I can have something like this? $(something).each( a=b; c=d; e=f); — Churk, Mar 23 '12 at 15:01
@Churk: No, you can't have that. That's what closures (anonymous functions) are for. — Ry-, Mar 23 '12 at 15:05
@minitech Ok, that make sense now. The point of having that anonymous functions and the added benefit of scoping. — Churk, Mar 23 '12 at 15:09

score 2 · Answer 1 · answered Mar 23 '12 at 14:53

2

This is not private function, it's just an anonymous function (function without name).

You could do:

$('input').each(function() {
 // do something
});

Or you could do:

function foo() {
 // do something
}
$('input').each(foo);

Anyway, the .each method needs a function to be the parameter which is a callback.

answered Mar 23 '12 at 14:53

xdazz

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the .each is just something I picked out as an example, but you are right, the .each() is a callback, but I have seem this on non-callback iterating function. – Churk Mar 23 '12 at 15:00

score 2 · Accepted Answer · answered Mar 23 '12 at 15:00

The anonymous function knows its current scope and variables.

function a() {
   var foo = 'test';
   $('something').each(function() {
      alert(foo);
   })
}

This function would alert 'test' because foo is defined within its scope.

function a() {
   var foo = 'test';
   $('something').each(b)
}

function b() {
   alert(foo);
}

This function would cause a javascript error stating that the variable foo is undefined.

If you need to define a function that only makes sense within an exact scope and don't want to throw lots of variables around or recalculate a lot of things, it's more comfortable to define an anonymous function.

score 1 · Answer 3 · answered Mar 23 '12 at 14:53

1

The first function is not private. It can reference all variables in the containing scope to form a closure, because JavaScript has lexical scoping.

The second function is pretty much the same thing, except the only thing you can do is provide it with a function name, or you will get a syntax error.

Short Answer: You put the function statement there to do more than one thing, or do one thing and provide parameters.

answered Mar 23 '12 at 14:53

cmc

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So the function allows you to pass in more parameter, such as $(a).each(function(c){ // do something with c}); where as I couldn't the other way. Ahhhh, thanks – Churk Mar 23 '12 at 14:58

Why user private function?

3 Answers3