How to get the nth value of a JavaScript generator?

Question

How can I get the nth value of a generator?

function *index() {
  let x = 0;
  while(true)
    yield x++;
}

// the 1st value
let a = index();
console.log(a.         


        

Answer 1

You could create an array of size n, and use Array.from and its second argument to fetch the values you need. So assuming iter is the iterator from generator gen:

var iter = gen();

Then the first n values can be fetched as follows:

var values = Array.from(Array(n), iter.next, iter).map(o => o.value)

...and when you are only interested in the n^th value, you could skip the map part, and do:

var value = Array.from(Array(n), iter.next, iter).pop().value

Or:

var value = [...Array(n)].reduce(iter.next.bind(iter), 1).value

The downside is that you still (temporarily) allocate an array of size n.