Set var to null on assignment if jQuery object is empty

Question

Is there a way to make a jQuery object evaluate to false when it's created if its length is 0?

Here's what I'd like to be able to do:

var Constructor = function (config) {

  this.obj = $(config.id) || null;

};

But that doesn't work -- if config.id is not passed or #id is not found, this.obj becomes an empty jQuery object that evaluates to true.

What I want is for this.obj to be null on creation (or to be a populated jQuery object if the arg/id exists) and to avoid having to check the length of the resulting jQuery object.

this.obj = $(config.id).get(0) may do it, it gets the Javascript object from the jQuery object — RealityDysfunction
– RealityDysfunction, Commented Jul 14, 2014 at 14:34
I think there is only one way to check if element exist by checking length of its array. — Dmitry Zaets
– Dmitry Zaets, Commented Jul 14, 2014 at 14:43

Pointy · Accepted Answer · 2014-07-14 14:42:36Z

2

You could wrap the .length test up in a jQuery function:

$.fn.nonEmpty = function() {
  return this.length && this;
};

Then:

this.obj = $(config.id).nonEmpty() || null;

answered Jul 14, 2014 at 14:42

Pointy

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5 Comments

cantera Over a year ago

Wouldn't this.obj have a value of true if it's not empty?

Pointy Over a year ago

@cantera no, because the JavaScript && operator returns the first non-false value without converting it to boolean. Thus, the return value will be either 0 or a non-empty jQuery object.

cantera Over a year ago

Thanks - I consider this the cleanest approach shown so far.

Pointy Over a year ago

Sorry that comment above explains thing incorrectly; && returns its first operand (left side) if that value is "falsy", and the second operand (right side) otherwise.

cantera Over a year ago

Thanks for clarifying -- this is a very useful technique.

Amit Joki · Accepted Answer · 2014-07-14 14:44:41Z

2

Just use its .length property:

var Constructor = function (config) {
  var elem = $(config.id);
  this.obj = elem.length ? elem : null;
};

Since jQuery returns an empty array if nothing is found, its length will be 0 which evaluates to false

If you want to avoid length lookup, you can do this:

var Constructor = function (config) {
  var elem = $(config.id);
  this.obj = elem[0] ? elem : null;
};

edited Jul 14, 2014 at 14:44

answered Jul 14, 2014 at 14:31

Amit Joki

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3 Comments

cantera Over a year ago

Updated my question -- I should have specified that checking the length in a separate step is what I'm trying to avoid.

cantera Over a year ago

Wasn't me - I'm only seeing the +2

Amit Joki Over a year ago

@cantera I know who it is and I also know it wasn't you :) It has been reverted back though

Dmitry Zaets · Accepted Answer · 2014-07-14 14:37:57Z

0

To make check if id is passed - you can use:

var Constructor = function (config) {

  this.obj = $(config.id || null );

};

But because of $ returns a collection - you should check length of this collection by .length

var Constructor = function (config) {    
  var $collection = $(config.id);

  this.obj = $collection.length ? $collection : null;
};

edited Jul 14, 2014 at 14:37

answered Jul 14, 2014 at 14:31

Dmitry Zaets

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4 Comments

Amit Joki Over a year ago

I suppose he wanted this.obj to be null. Your code returns [] which is not what OP wants I suppose

cantera Over a year ago

Good idea, but that produces the same result. If I evaluate this.obj, it still returns true even if null is being wrapped.

cantera Over a year ago

@AmitJoki: Yes that's correct, I'd like this.obj to take the value null.

Amit Joki Over a year ago

@cantera check out my answer then. It does what you want.

RealityDysfunction · Accepted Answer · 2014-07-14 15:23:48Z

0

As per my comment, another short solution is this:

var check = $(config.id);
this.obj = check.get(0)==null ? null : check

Look here: http://jsfiddle.net/V3G6U/2/

edited Jul 14, 2014 at 15:23

answered Jul 14, 2014 at 15:07

RealityDysfunction

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2 Comments

cantera Over a year ago

Thanks - I like the syntax, but unfortunately it requires diving into the DOM twice to find the element.

RealityDysfunction Over a year ago

Fixed, its a 2 liner now ;p.

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