Are node.js arrays actually hashmaps?

Question

To my surprise, this code actually works in node.js:

var arr = new Array();
// also works: var arr = [];
arr[0] = 123;
arr['abc'] = 456;
arr; // node.js: [ 123, abc: 456 ], chrome: [123]

I've always thought that an array stores its objects in order, only accessible by an integer key, like a std::vector in C++. However, here it's acting like a map or an object. Adding to the confusion, the same code works as expected in chrome, returning an array with a single entry, 123. I thought node.js and chrome javascript use the same internal engine, V8. What's going on here?

Answer 1

Javascript allows you to extend objects on the fly, and as an Array is an object you can do so.

What you are doing there is adding a new property to your array called abc and assigning it the value 456.

So you could say every object in Javascript can be used as a hashmap somehow.

EDIT

It seems that Chrome filters the non-numeric properties of the Array object at dumping whilst Node dumps every user-defined property. In my opinion Node's way is better since the alpha-numeric property is available in a for in statement:

var a = [1];
a['abc'] = 2;
for (var i in a) {
    console.log(i);
}
// Prints:
// 0
// abc

Answer 2

The answers are right, the behaviour is maybe more understandable, if you try to display length of the array.

var ar = []
ar[0] = 42
console.log(ar.length) // 1

ar[12] = 21
console.log(ar.length) // 13

ar['ab'] = 1
console.log(ar.length) // 13 (separate property, not in array)

ar[ar.length] = 33
console.log(ar.length) // 14

ar.push(55)
console.log(ar.length) // 15

console.log(ar) // display all items specified above
//[ 42, , , , , , , , , , , , 21, 33, 55, ab: 1 ]
// which in fact really is:
// [ 42, , , , , , , , , , , , 21, 33, 55] as array and 
// special property of array object - 'ab':1