Question

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What is the difference between an array and an object?
The item exists in the array but it says that the array is 0 length?

I m a bit confused in object and associative array in javascript. I read this :question but this question says that there is not much of a difference in both. I wrote this in the console:

var a = [];
a["A"] = 1;

var b = {};
b["B"] = 2;

var c = new Object();
c["C"] = 3;

output for above are as:

a gives {A : 1} 
b gives {B : 2}
c gives {C : 3}

All above three cases gives same reault as in they all gives an object. Question is how all above 3 cases are treated in javascript.

Answer 1

In your example, b and c are essentially the same thing, because {} is the equivalent of new Object().

Coming back to a, it’s defined as an Array which is a special kind of Object in the sense that numeric properties (based on uint32) are treated differently. Its length property gets updated when those properties are added and removed.

When you use 'A' as an index, it gets treated as a regular property, defined by how Object works with properties, so you can access it as a.A or a['A'] whereas an index of [5] can only be accessed using a[5].

Normally, the debug output of an array is always [] unless the length property is non-zero, so the output you’ve shown is somewhat irregular.

Trivia

According to the ECMAScript documentation, a particular value p can only be an array index if and only if:

(p >>> 0 === p) && (p >>> 0 !== Math.pow(2, 32) - 1)

See also:

The item exists in the array but it says that the array is 0 length?