Why doesn't `new Array(new Number(3))` produce an array of length 3?

Question

When I write new Array(new Number(3)) in the console, it produces an array of length 1 with that item being a Number object with valueOf=3. Why does the array constructor treat new Number(3) different to the primitive version of 3?

1. Can someone link me to a standards doc where this Array behaviour is specified?
2. Is this something that I need to watch out for in general, or is it specific to the Array constructor?

Thanks!

Answer 1

Because in new Array(new Number(3)), the parameter new Number(3) returns an object and not the number.

Where as new Array(Number(3)) will work as Number(3) returns 3, the number.

Thanks to @Andreas' comment, here's the specification: ECMAScript 22.1.1.2 Array (len)

Step 7: "If Type(len) is not Number, then

1. Let defineStatus be CreateDataProperty(array, "0", len).
2. Assert: defineStatus is true.
3. Let intLen be 1.

Answer 2

From the MDN

If the only argument passed to the Array constructor is an integer between 0 and 2³²-1 (inclusive), this returns a new JavaScript array with its length property set to that number

Instance of new Number(3) is not an integer(number).

Number.isInteger(3) // -> true

Number.isInteger(Number(3)) // -> true

Number.isInteger(new Number(3)) // -> false