add iterators generators lecture

app/index.js

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+// // part 1:
+// const arrayIterator = (array) => {
+//   let index = 0;
+//
+//   return {
+//     next: () => {
+//       if (index < array.length) {
+//         let next = array[index];
+//         index += 1;
+//         return next;
+//       }
+//     }
+//   }
+// }
+//
+// var it = arrayIterator([1, 2, 3]);
+// console.log(it.next());
+// console.log(it.next());
+// console.log(it.next());
+// console.log(it.next());
+
+// // part 2:
+// function* arrayIterator() {
+//   yield arguments;
+// }
+//
+// var it = arrayIterator(1, 2, 3);
+// console.log(it.next().value);
+
+// // part 3:
+// function* arrayIterator() {
+//   for (let arg of arguments) {
+//     yield arg
+//   }
+// }
+//
+// var it = arrayIterator(1, 2, 3);
+//
+// // part 4:
+// function* arrayIterator() {
+//   yield* arguments;
+// }
+//
+// var it = arrayIterator(1, 2, 3);
+//
+// // part 5:
+// var array = [1, 2, 3];
+// var it = arrayIterator(...array);
+//
+// // parts 2-5:
+// console.log(it.next().value);
+// console.log(it.next().value);
+// console.log(it.next().value);
+// console.log(it.next().value);

app/script.txt_iterators_generators

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+// TODO add to google doc.
+
+console.log(it.next().value);
+console.log(it.next().value);
+console.log(it.next().value);
+console.log(it.next().value); With some console.logs to it now
+we find that the iterator yields each argument within arguments
+individually.
+
+Note that we've essentially used the yield keyword 3 times in this loop
+and es6 provides a shortcut for us whenever we use yield more than one time in a row.
+We can just use the yield asterisk syntax and reform the for loop to one line with:
+  yield* arguments; And you'll notice in our console that everything works the same!
+
+Great! now to transform the arguments in arrayIterator to a true array and fully mimic
+the iterator we created originally we could just pass a spread array rather than arguing each
+element one a a time, and boom we have a fully mimicked iterator with es6 generators.
+
+Awesome! This covered a more advanced topic within JavaSCript and es6 in general.
+And if you plan on coding in languages in more than just JS and ES6, then iterators will show up everywhere.
+Most of all though, generators will allow you to approach more advanced programs in ES6,
+expecially with the convenient yield keyword and next method!