fibonacci_generator.js

// A generator function for generating the first n number(s)
// of the Fibonacci sequence.
// Yields numbers as BigInt.
function* fibonacci(n) {
  let i = 0;
  
  n = (Number.isInteger(n) && n > 0) ? n : Infinity;
  
  for (; i < 2 && i < n; i++) {
    yield 1n;
  }
  
  if (n > 2) {
    let [a, b] = [1n, 1n];
    
    for (; i < n; i++) {
      [a, b] = [b, a + b];
      yield b;
    }
  }
}

Nice!