iterators-generators.js

/****************************************
ES6 Iterators and Generators
Arrays, Strings, Maps, Sets, NodeLists - built-in iterators

{Object} => Iterator => Generator

****************************************/
let log = console.log;
let characters = ['Finn','Poe', 'Rey', 'Kylo', 'Luke', 'Leia']

function* genny(){
    let i = 0;
   yield characters[i];
    i++;
    yield characters[i];
    i++;
    yield characters[i];
    i++;
    yield characters[i];
    i++;
    yield characters[i];
    i++;
    yield characters[i];
    i++;
    yield characters[i];
    i++;
    yield characters[i];
}

//let iter = genny();
//log(iter)
//log(iter.next());
//log(iter.next());
//log(iter.next());
//log(iter.next());
//log(iter.next());
//log(iter.next());
//log(iter.next());
//log(iter.next());
//log(iter.next());


let starwars8 = {
    title: 'The Last Jedi',
    director: 'Rian Johnson',
    year: 2017,
    boxOffice: '1.3B'
}


let count = -1;
let SW8 = {
    [Symbol.iterator]: function(obj){
        return {
            next: ()=>{
                count++;
                switch(count) {
                    case 0:
                        return {
                            value: obj.title, 
                            done: false}
                    case 1:
                        return {
                            value: obj.year,
                            done: false
                        }
                    case 2:
                        return {
                            value: obj.director,
                            done: false
                        }
                    case 3 :
                        return { value: undefined, done: true}
                    default:
                        return { value: undefined, done: true}
                }
            }
        }
    }
}

let data = SW8[Symbol.iterator](starwars8);
log( data.next() );
log( data.next() );
log( data.next() );
log( data.next() );
log( data.next() );

//
//for(let p of starwars8){
//    for of loops are using the .next( ) method behind the scenes
//}

version-2.js

/**
This second version has two objects - `Obj1` and `Obj2`
The generator `myGenerator`, is put in the prototype chain for both objects
This way we can call the [Symbol.iterator] method on each object to give them each their own iterator.
The new version of the Iterator also checks for the existence of the property, to make this more generic. It will
display a message about missing properties.
**/
let log = console.log;

let Obj1 = {
  title: 'The Last Jedi',
  director: 'Rian Johnson',
  year: 2017,
};

let Obj2 = {
  title: 'The Hobbit',
  director: 'Peter Jackson',
  boxOffice: '1.017B',
};

let myGenerator = {
  [Symbol.iterator]: function () {
    let count = -1; //reset the count
    return {
      next: () => {
        count++;
        switch (count) {
          case 0:
            return {
              value: this.title ? this.title : 'No title property',
              done: false,
            };
          case 1:
            return {
              value: this.year ? this.year : 'No year property',
              done: false,
            };
          case 2:
            return {
              value: this.director ? this.director : 'No director property',
              done: false,
            };
          default:
            return { value: undefined, done: true };
        }
      },
    };
  },
};

Object.setPrototypeOf(Obj1, myGenerator);
Object.setPrototypeOf(Obj2, myGenerator);

let data1 = Obj1[Symbol.iterator](); //create the iterator for Obj1
//this sets count to -1
for (let prop of Obj1) {
  log(prop);
}
log('\n\n');

let data2 = Obj2[Symbol.iterator](); //create the iterator for Obj2
//this sets count to -1 again
for (let prop of Obj2) {
  log(prop);
}

Hi Steve!
Quick question regarding creating the iterators for Obj1 and Obj2 ('data1' and 'data2' respectively).
I'm unsure of what's happening "behind the scenes" inside the for/of loops when we create each object's iterators in those variables.

//From your code snippet:

let data1 = Obj1Symbol.iterator;
for (let prop of Obj1){
log(prop);
}

// Is this the equivalent of what's happening with the 'data1' variable behind the scenes in that for/of loop?

let data1 = Obj1Symbol.iterator;
for (let a = 0; a < Object.keys(Obj1).length; a++){
log(data1.next())
}

Thanks for all your hard work!
-J

Basically yes.
It is also checking to see if done === false each time after calling next()