Question

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?

Answer 1

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