js_yield_generator_fibonacci.js

/*
From: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Iterators_and_Generators

Generators compute their yielded values on demand, which allows them to efficiently 
represent sequences that are expensive to compute, or even infinite sequences as 
demonstrated above.

The next() method also accepts a value which can be used to modify the internal state
of the generator. A value passed to next() will be treated as the result of the last 
yield expression that paused the generator.

Here is the fibonacci generator using next(x) to restart the sequence:

*/

function* fibonacci() {
  var fn1 = 0;
  var fn2 = 1;
  while (true) {  
    var current = fn1;
    fn1 = fn2;
    fn2 = current + fn1;
    var reset = yield current;
    if (reset) {
        fn1 = 0;
        fn2 = 1;
    }
  }
}

var sequence = fibonacci();
console.log(sequence.next().value);     // 0
console.log(sequence.next().value);     // 1
console.log(sequence.next().value);     // 1
console.log(sequence.next().value);     // 2
console.log(sequence.next().value);     // 3
console.log(sequence.next().value);     // 5
console.log(sequence.next().value);     // 8
console.log(sequence.next(true).value); // 0
console.log(sequence.next().value);     // 1
console.log(sequence.next().value);     // 1
console.log(sequence.next().value);     // 2