Question

95

rated 0 times [ 97] [ 2] / answers: 1 / hits: 49682 / 12 Years ago, tue, august 21, 2012, 12:00:00

I'm having an interesting issue that I'm sure is easily explained, but the explanation is eluding me.

An undefined or null object in javascript is equal to false.

var x;

alert(!x); //returns true

alert(x==true); //returns false

What about an empty array object? Is that the equivalent of true or false?

var x = [];

alert (x==true); //returns false

alert (!x); //returns false

If it is equivalent to true, how do I check if it's non-empty? I was hoping to do

if (!x) {

    //do stuff

}

I tried checking x.length, but I'm using this object as a map:

var x = [];

alert(x.length); //returns 0

x.prop = hello; 

alert(x.length); //still returns 0

How can I check if my map is empty?

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4 Years ago

answered 12 Years ago jaelyn · Accepted Answer

Little confused as you seem to be mixing objects and arrays in your question. Hopefully this might help clear it up for you.

An empty array evaluates to true:

!![] //true

But integer 0 evaluates to false, so you can do:

!![].length //false

So:

if([].length) {

   //array has 1 element at least

} else {

  //array has 0 elements

}

However you do seem to be getting arrays and objects confused. In JavaScript, we have objects:

var x = {};

x.foo = bar;

x.baz = 2;

And we have arrays:

var x = [];

x.push(foo);

x.length //1

You can't do what you do in your opener:

var x = []; //x is an array

x.foo = bar; //can't do this on an array

x.length; // 0