ES5 generator like ES6 yeald & *foo

es5-genarator.js

function foo() {
  function nextState(v) {
    switch (state) {
      case 0:
        state++;
        return 42; // yield
      case 1:
        state++;
        x = v; // yield ended
        console.log(x);
        return undefined;
      // not need state 2  
    }
  }
  
  var state = 0, x;
  
  return {
   next: function(v) {
     var ret = nextState(v);
     return {value: ret, done: (state === 2)};
   }
    // missed method return() & throw()
  }
}

var it = foo();
it.next(); // {value: 42, done: false}
it.next(10); // {value: undefined, done: true}

///
function *foo() {
  yield 1;
  yield 2;
  yield 3;
}

for (var v of foo()) { // need iterator
  console.log(v); 
}
// 1 2 3

var it = foo();
it.next(); // {value: 1, done: false}
it.next(); // {value: 2, done: false}
it.next(); // {value: 3, done: false}
it.next(); // {value: undefined, done: true}