Generator Return Values are Weird

gistfile1.js

window.console.clear();

function* gen1() {
  yield 1;
  yield 2;
  return 3;
}

function printForOf(iter) {
  for(var x of iter) {
    console.log(x);
  }
}

function printTilDone(iter) {
  var result;
  while(true) {
    result = iter.next();
    console.log(result);
    if(result.done) {
      return;
    }
  }
}

printForOf(gen1());
// 1
// 2

printTilDone(gen1());
// { value: 1, done: false }
// { value: 2, done: false }
// { value: 3, done: true }


/**
 * what about merging two?
 * */
 
 function* gen2() {
   yield 'a';
   yield 'b';
   return 'c';
 }
 
 function* concat(...iters) {
   for(let iter of iters) {
     for(let x of iter) {
       yield x;
     }
   }
 }

printForOf(concat(gen1(), gen2()));

// 1
// 2
// a
// b

/**
 * but what to do with the returns? meh.
 * */
 
 
function* otherConcat(...iters) {
  var returnCollection = [];
  for(let iter of iters) {
    while(true) {
      let result = iter.next();
      if(result.done) {
        // What do I do here? Collect them? Why?
        // what if one of them doesn't have a return value? 
        // I just use `undefined`?
        returnCollection.push(result.value);
        break;
      } else {
        yield result.value;
      }
    }
  }
  return returnCollection;
}

printForOf(otherConcat(gen1(), gen2()));
// 1
// 2
// 'a'
// 'b'

printTilDone(otherConcat(gen1(), gen2()));
// { value: 1, done: false }
// { value: 2, done: false }
// { value: 'a', done: false }
// { value: 'b', done: false }
// { value: [3, 'c'], done: true }