Mastering JavaScript

Chapter 7
The keys() and values() iterators both return a list of the unique values in the Set.
The entries() iterator yields a list of entry arrays, where both items of the array are
the unique Set values. The default iterator for a Set is its values() iterator.
Symbols
ES6 introduces a new data type called Symbol. A Symbol is guaranteed to be unique
and immutable. Symbols are usually used as an identifier for object properties.
They can be considered as uniquely generated IDs. You can create Symbols with the
Symbol() factory method—remember that this is not a constructor and hence you
should not use a new operator:
let s = Symbol();
console.log(typeof s); //symbol
Unlike strings, Symbols are guaranteed to be unique and hence help in preventing
name clashes. With Symbols, we have an extensibility mechanism that works for
everyone. ES6 comes with a number of predefined built-in Symbols that expose
various meta behaviors on JavaScript object values.
Iterators
Iterators have been around in other programming languages for quite some time.
They give convenience methods to work with collections of data. ES6 introduces
iterators for the same use case. ES6 iterators are objects with a specific interface.
Iterators have a next() method that returns an object. The returning object has two
properties—value (the next value) and done (indicates whether the last result has
been reached). ES6 also defines an Iterable interface, which describes objects that
must be able to produce iterators. Let's look at an array, which is an iterable, and the
iterator that it can produce to consume its values:
var a = [1,2];
var i = a[Symbol.iterator]();
console.log(i.next()); // { value: 1, done: false }
console.log(i.next()); // { value: 2, done: false }
console.log(i.next()); // { value: undefined, done: true }
As you can see, we are accessing the array's iterator via Symbol.iterator() and
calling the next() method on it to get each successive element. Both value and done
are returned by the next() method call. When you call next() past the last element
in the array, you get an undefined value and done: true, indicating that you have
iterated over the entire array.
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