Array.length gives incorrect length

Question

If I have an array having object as values at the indices like:

var a = [];
a[21] = {};
a[90] = {};
a[13] = {};
alert(a.length); // outputs 91

Answer 1

The array in JavaScript is a simple zero-based structure. The array.length returns the n + 1 where n is the maximum index in an array.

That's just how it works - when you assign 90'th element and this array's length is less than 90, it expands an array to 90 and sets the 90-th element's value. All missing values are interpreted as null.

If you try the following code:

var a = [];
a[21] = {};
a[90] = {};
a[13] = {};
console.log(JSON.stringify(a));

You will get the following JSON:

[null,null,null,null,null,null,null,null,null,null,null,null,null,{},null,null,null,null,null,null,null,{},null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,{}]

Moreover, array.length is not a readonly value.
If you set a length value less than the current, then the array will be resized:

 var arr = [1,2,3,4,5];
 arr.length = 3;
 console.log(JSON.stringify(arr));
 // [1,2,3]

If you set a length value more than the current, then the array will be expanded as well:

 var arr = [1,2,3];
 arr.length = 5;
 console.log(JSON.stringify(arr));
 // [1,2,3,null,null]

In case you need to assign such values, you can use JS objects.
You can use them as associative array and assign any key-value pairs.

var a = {};
a[21] = 'a';
a[90] = 'b';
a[13] = 'c';
a['stringkey'] = 'd';
a.stringparam = 'e'; // btw, a['stringkey'] and a.stringkey is the same

console.log(JSON.stringify(a)); 
// returns {"13":"c","21":"a","90":"b","stringkey":"d","stringparam":"e"}

console.log(Object.keys(a).length);
// returns 5