Fibonacci sequence - javascript generator

gistfile1.txt

function * fibon () {
 let val1 = 0; 
 let val2 = 1;
 let swap;
 yield val1;
 yield val2;
 while (true) {
  swap = val1 + val2;
  val1 = val2;
  val2 = swap;
  yield swap;
 } 
}

// Usage like:
let fib = fibon();
console.log(fib.next()); // 0
console.log(fib.next()); // 1
console.log(fib.next()); // 1
console.log(fib.next()); // 2
console.log(fib.next()); // 3
...