What's the difference between Array(1) and new Array(1) in JavaScript?

Question

I just started thinking about this, but couldn't get any differences to expose themselves whilst mucking around in jsFiddle.

var a = new Array(1),     b = Array(1);  console.log(a, b); 

Output is two arrays with one undefined member.

Doing a for ( in ) reveals they have the same properties.

What are the differences between these? Does the first one simply instantiate the object explicitly?

Please don't lecture me about using array literal notation as I already know about that. I'm more wishing to fill this gap in my knowledge explained above.

Answer 1

With Array, both are equivalent. The new is injected when it's called as a function:

15.4.1 The Array Constructor Called as a Function

When Array is called as a function rather than as a constructor, it creates and initialises a new Array object. Thus the function call Array(…) is equivalent to the object creation expression new Array(…) with the same arguments.

From ECMA-262, 3th Edition (with similar in 5th Edition). See also 22.1.1 The Array Constructor in ECMA-262 ECMAScript 2020 specification (11th Edition).

Answer 2

According to Javascript: The Definitive Guide (5th Edition), page 602, "When the Array() constructor is called as a function, without the new operator, it behaves exactly as it does when called with the new operator."