what does Array do in this Javascript function?

I have seen a bit of code similar to this demonstration purpose function, somewhere:

function Pract() {
   let a = Array, b;
   b = 1;
   return b;
}
console.log(Pract())
// Output: 1

However, when I remove Array from the a variable it shows an error:
// And I know this is because b is not defined, but so does b in let a = Array, b;

function Pract() {
    let a = b;
    b = 1;
    return b;
}
console.log(Pract())
// Output: Error

Thanks in advance.