Difference between a = [undefined] and a = new Array(1)

Question

var a = [undefined, undefined];
var b = new Array(2);

0 in a // returns true
1 in a // returns true
0 in b // returns false
1 in b // returns false

Can anyone explain to me why this is happening?

Answer 1

b = new Array(2);

just says that, b is an array of size two. But the array is just empty. It doesn't have any elements in it. So, there are no indexes as such. So, 0 and 1 are not yet there in b. These are called holes.

But, when you say

var a = [undefined, undefined];

you are creating an array with two elements in it. So, it has undefined at index 0 and 1. That is why they both exist in the array.

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Note: When you simply assign a value to a variable, without declaring it with var (let or const), it will become a global variable.