CHAPTER 12
Iterators and Generators
ES6 introduces two very important new concepts: iterators and generators. Generators
depend on iterators, so we’ll start with iterators.
An iterator is roughly analogous to a bookmark: it helps you keep track of where you
are. An array is an example of an iterable object: it contains multiple things (like pages
in a book), and can give you an iterator (which is like a bookmark). Let’s make this
analogy concrete: imagine you have an array called
book
where each element is a
string that represents a page. To fit the format of this book, we’ll use Lewis Carroll’s
“Twinkle, Twinkle, Little Bat” from Alice’s Adventures in Wonderland (you can imag‐
ine a children’s book version with one line per page):
const
book
=
[
"Twinkle, twinkle, little bat!"
,
"How I wonder what you're at!"
,
"Up above the world you fly,"
,
"Like a tea tray in the sky."
,
"Twinkle, twinkle, little bat!"
,
"How I wonder what you're at!"
,
];
Now that we have our
book
array, we can get an iterator with its
values
method:
const
it
=
book
.
values
();
To continue our analogy, the iterator (commonly abbreviated as
it
) is a bookmark,
but it works only for this specific book. Furthermore, we haven’t put it anywhere yet;
we haven’t started reading. To “start reading,” we call the iterator’s
next
method,
which returns an object with two properties:
value
(which holds the “page” you’re
now on) and
done
, which becomes
false
after you read the last page. Our book is
only six pages long, so it’s easy to demonstrate how we can read it in its entirety:
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