Regarding the calling mechanism of jquery function

Question

Suppose I have created one jQuery function to check if elements exist or not like:

jQuery.fn.exists = function () { 
    return this.length > 0; 
}

Then I call:

if ($("#MyDiv").exists() == false)
   alert('not exist');
else
   alert('exist');

If I call jQuery function above it works. But can't we call the jquery function like this way exists('#MyDiv') ? If I try to call this way then I am not getting result...why?

Unless you have defined a function `exists`, you cannot call it (obviously?). Of course you can easily define one which accepts a selector and performs the same operation as you defined in `$.fn.exists`. Or am I missing something? — Felix Kling, Jun 19 '12 at 18:43
@FelixKling. What all of we're missing, what this function is good for... — gdoron, Jun 19 '12 at 18:44
@gdoron: I agree with you that such a method is rather unnecessary in this particular case, but the question still stands (imo) apart from what the method is doing. — Felix Kling, Jun 19 '12 at 18:45
@Thomas, Are you interested in the `exist` function only, or are you asking a more broad question? — gdoron, Jun 19 '12 at 18:48

score 2 · Answer 1 · edited Jun 19 '12 at 18:52

2

ARRG, please, don't use this useless exist function!

It can be simply with:

if ($('#MyDiv').length)
    // Exist
else
    // Doesn't exist.

No plugins needed, everyone knows what this code does, Don't use exist.

edited Jun 19 '12 at 18:52

gen_Eric

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answered Jun 19 '12 at 18:40

gdoron

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I don't think that's point of the question. I assumed `exist` was just an example. – gen_Eric Jun 19 '12 at 18:44
2

@Rocket. I don't think so, look at [this question](http://stackoverflow.com/q/31044/601179) see how many people like that useless function. – gdoron Jun 19 '12 at 18:46
"Syntactic sugar causes cancer of the semicolon." -Alan Perlis – gen_Eric Jun 19 '12 at 18:50
1

@Rocket. Actually the main problem with a lot of the jQuery programmers, they have **ZERO** knowledge in javascript, so they didn't thought about `if($(...).length)` instead of `$(...).length > 0` – gdoron Jun 19 '12 at 18:52
@gdoron: There is nothing wrong with being explicit (`> 0`). This can make the code easier to understand. But I guess this is rather a matter of style. `if($(...))` is worse ;) – Felix Kling Jun 19 '12 at 18:55
1

@FelixKling, I expect every programmer ever touching\reading my real code to know about truthy-falsy nature of javascript values, so `> 0` is just noise. IMO. – gdoron Jun 19 '12 at 18:57

Selvakumar Arumugam · Accepted Answer · 2012-06-19T18:46:32.960

1

Try like below,

$.exists = function (selector) {
    return $(selector).length > 0;
}

and use as,

$.exists('#test')

DEMO: http://jsfiddle.net/skram/hgaPt/1/

Please use this in a bigger picture, using it for simple thing as exist is just a overkill.

edited Jun 19 '12 at 18:46

answered Jun 19 '12 at 18:40

Selvakumar Arumugam

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why u did not specify jquery function prototype like jQuery.fn.exists = function () {) rather u wrote code like $.exists = function (selector) {)....both are same? jQuery.fn keyword is optional. please discuss – Thomas Jun 20 '12 at 07:01

score 0 · Answer 3 · answered Jun 19 '12 at 18:46

exists('#MyDiv')

This won't work because you didn't make a function called exists. You made a function called jQuery.fn.exists (jQuery.fn is the prototype of jQuery objects).

For exists('#MyDiv') to work, you need to make a function called exists.

function exists(sel){
    return $(sel).length;
}

score 0 · Answer 4 · edited Jun 19 '12 at 18:48

0

$.fn is equal to $.prototype, so indirectly using $.fn you are hooking your functions to jQuery object prototype, so the selector needs to be passed to the jQuery object not the chained function name.

edited Jun 19 '12 at 18:48

gen_Eric

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answered Jun 19 '12 at 18:47

Ajay Beniwal

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Regarding the calling mechanism of jquery function

4 Answers4