Mastering JavaScript

Data Structures and Manipulation
We discussed briefly that JavaScript arrays are not really arrays in a traditional sense.
In JavaScript, arrays are objects that have the following characteristics:
• The length property
• The functions that inherit from Array.prototype (we will discuss this in the
next chapter)
• Special handling for keys that are numeric keys
When we write an array index as numbers, they get converted to strings—arr[0]
internally becomes arr["0"]. Due to this, there are a few things that we need to be
aware of when we use JavaScript arrays:
• Accessing array elements by an index is not a constant time operation as it is
in, say, C. As arrays are actually key-value maps, the access will depend on
the layout of the map and other factors (collisions and others).
• JavaScript arrays are sparse (most of the elements have the default value),
which means that the array can have gaps in it. To understand this, look at
the following snippet:
var testArr=new Array(3);
console.log(testArr);
You will see the output as [undefined, undefined, undefined]—
undefined is the default value stored on the array element.
Consider the following example:
var testArr=[];
testArr[3] = 10;
testArr[10] = 3;
console.log(testArr);
// [undefined, undefined, undefined, 10, undefined, undefined,
undefined, undefined, undefined, undefined, 3]
You can see that there are gaps in this array. Only two elements have elements
and the rest are gaps with the default value. Knowing this helps you in a couple of
things. Using the for...in loop to iterate an array can result in unexpected results.
Consider the following example:
var a = [];
a[5] = 5;
for (var i=0; i