JavaScript

6Iterables, Iterators, for-of, Iterable Spread, Generators

WHAT'S IN THIS CHAPTER?

• Iterators and Iterables
• for-of loops
• Iterable spread syntax
• Generators and generator functions

CODE DOWNLOADS FOR THIS CHAPTER

You can download the code for this chapter at https://thenewtoys.dev/bookcode or https://www.wiley.com/go/javascript-newtoys.

JavaScript got a new feature in ES2015: generalized iteration, the classic “for each” idiom popular in many languages. In this chapter you'll learn about the new feature's iterators and iterables and how you create and use them. You'll also learn about JavaScript's powerful new generators that make creating iterators easy, and also take the concept further, making communication two-way.

ITERATORS, ITERABLES, THE FOR-OF LOOP, AND ITERABLE SPREAD SYNTAX

In this section, you'll learn about JavaScript's new iterators and iterables, the for-of loop that makes using iterators easy, and the handy spread syntax ( …) for iterables.

Iterators and Iterables

Many languages have some kind of “for each” construct for looping through the entries of an array or list or other collection object. For years, JavaScript only had the for-in loop, which isn't a general-purpose tool (it's just for looping through the properties of an object). The new Iterator and Iterable “interfaces” (see note) and new language constructs that use them change that.

NOTE Starting with ES2015, the specification mentions “interfaces” (in the sense of code interfaces) ...