It's funny, I was going to suggest the same thing, right down to the method
name, but Hamish beat me to it. :-)
It's interesting that people don't immediately think of using
$('#id').length > 0 (with or without the > 0). It tells me that they're not
aware of a fundamental fact about jQuery: The $() function always returns an
array-like object that has .length and [0..n] properties just like any Array
- even when you use an ID selector. Once you understand that, a lot of
things fall into place. Maybe the documentation should emphasize this point
more.
Consider the corresponding question for a string: "Does the string have any
characters in it?"
You could code:
if( string.match(/.+/) ) { alert('The string has text'); }
But you're more likely to write:
if( string.length > 0 ) { alert('The string has text'); }
Of course, every JavaScript programmer should acquire some familiarity with
regular expressions, and should know why .match(/.+/) works - but you
probably wouldn't use it when a simple .length test is sufficient.
Anyway, Hamish's .exists() function does seem like the best of both worlds.
A couple of little improvements: It doesn't need to be defined inside a
$(document).ready(), and as long as we're encapsulating the logic anyway,
it's better to use the more efficient .length>0 test. So, the code would be:
jQuery.fn.exists = function() {
return this.length > 0;
};
There are some situations where you could use .exists() or .is('*') or
.length>0 but there is a better alternative. One case would be where you
want to access a DOM property directly, so you need to know that the element
actually exists before you do that:
if( $('#id').exists() )
alert( $('#id')[0].tagName );
It would be much better to write it this way:
var element = $('#id')[0];
if( element )
alert( element.tagName );
-Mike
> From: McLars
>
> Hamish, I like that!
>
> Michael, the question is number two on the FAQ. Despite what
> seasoned programmers may think, it's obviously not intuitive
> to newbies.
>
>
> Personally, I think it's always good to point out
> alternatives to learners. The .is() method is actually quite
> versatile and can be used to test for more than just
> existence. It doesn't hurt to keep an open mind and look at
> all the alternatives--even for something as simple as
> checking existence. I didn't even know about the .is() method before.
> I looked in the docs, tried it out, and actually learned something.
>
> Larry
>
>
> On Dec 26, 3:08 pm, Hamish Campbell <[EMAIL PROTECTED]> wrote:
> > extendify!
> >
> > $(document).ready(function() {
> > jQuery.fn.exists = function() {
> > return ( this.is('*') )
> > }
> > }
> >
> > });
> >
> > On Dec 27, 10:28 am, "Michael Geary" <[EMAIL PROTECTED]> wrote:
> >
> >
> >
> > > It's funny, if I saw:
> >
> > > if( $('#id').is('*') ) { ... }
> >
> > > I would have no clue what the code was trying to do until
> I thought
> > > hard about it: "Let's see... is star... Now that's going
> to match *anything*.
> > > Wouldn't it always return true? Naw, that can't be right,
> what would
> > > be the point of this code... Oh! What if there are no elements at
> > > all? Then it would return false! I think it would anyway.
> Better check the docs. Hmm...
> > > The docs don't explicitly say what .is() does when the
> array is empty.
> > > Better check the source code, and maybe try a couple of
> test cases
> > > to be sure."
> >
> > > Where if I saw:
> >
> > > if( $('#id').length ) { ... }
> >
> > > I would know right away what it does: "$() returns an
> array. Does it
> > > have any elements?" :-)
> >
> > > -Mike
> >
> > > > From: McLars
> >
> > > > $('#id').length is the old school, and most widely
> used, technique.
> > > > This is probably the fastest.
> >
> > > > $('#id').is('*') does make sense semantically (expresses the
> > > > intent), and is more flexible.
> > > > > From: Alexey Blinov
> >
> > > > > Yep... my code have size(). Forgot to point it...
> > > > > And thanks for info about more efficient way - using length.
> > > > > So... which way is better than?
> >
> > > > > 1. $('#id').length > 0
> >
> > > > > 2. $('#id').length() !== 0
> >
> > > > > 3. $('#id').is('*') //never try it... but look pretty- Hide
> > > > > quoted text -
> >
> > > - Show quoted text -- Hide quoted text -
> >
> > - Show quoted text -
>
