myArray.length in ECMAScript 262

The number of items in the array.

Property Type

This property returns an integer value that is one greater than the largest index in the array. As arrays are sparse, it is mostly a convenience property for iterating over the specified values in an array. Consider the following code:

var foo = [];
// ** foo.length is 0
// ** foo[10] is undefined -- typeof(foo[10])=='undefined'

// ** foo.length is now 2

foo[6] = "Zebras";
// ** foo.length is now 10
// ** foo[5] is undefined, despite being 'in' the array

// ** foo[0] is now undefined, having previously been "Hello"

In the above, we see that:

Note that it is almost always (slightly) faster to assign the number of items in an array to a local variable when iterating the array, rather than looking up the length on each iteration:

for (var i=0,len=myArray.length;i<len;i++){ ... }