Confused by behavior of `map` on arrays created using `new` [duplicate]

Answer 1

One additional answer on the behavior of console.log. Plain simple, the output is sugar and technically wrong.

Lets consider this example:

var foo = new Array(4),
    bar = [undefined, undefined, undefined, undefined];

console.log( Object.getOwnPropertyNames(bar) );
console.log( Object.getOwnPropertyNames(foo) );

As we can see in the result, the .getOwnPropertyNames function returns

["length"]

for the foo Array/Object, but

["length", "0", "1", "2", "3"]

for the bar Array/Object.

So, the console.log just fools you on outputting Arrays which just have a defined .length but no real property assignments.