javascript - In ES6, what happens to the arguments in the first call to an iterator's `next` method?

Question

If you have an generator like,

function* f () {
  // Before stuff.
  let a = yield 1;
  let b = yield 2;
  return [a,b];
}

And, then run

var g = f();
// this question is over this value.
g.next(123); // returns: { value: 1, done: false }
g.next(456); // returns: { value: 2, done: false }
g.next(); // returns: { value: [ 456, undefined ], done: true }

The first call to .next() to set a to 123 and the second call to set b to 456, however at the last call to .next() this is return,

{ value: [ 456, undefined ], done: true }

Does the argument in the first call to g.next get lost? What happens to them? Using the above example, how do I set a?

Answer 1

Try:

var g = f();
// this question is over this value.
g.next(); // returns: { value: 1, done: false }
g.next(123); // returns: { value: 2, done: false }
g.next(456); // returns: { value: [123, 456], done: true }