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Iterators

Iterators

Introduction

The iteration protocol defines a standard way to produce a sequence of values. An iterable has a Symbol.iterator method that returns an iterator (an object with a next() method).

Learning Objectives

Understand the iteration protocol (iterable + iterator)
Use for...of with built-in iterables
Create custom iterables with Symbol.iterator
Know which built-in types are iterable

Visual Summary

sequenceDiagram
    participant Loop as for...of
    participant Iterable as Array [1,2,3]
    participant Iterator as Iterator

    Loop->>Iterable: Symbol.iterator
    Iterable->>Iterator: Return iterator
    Loop->>Iterator: next()
    Iterator->>Loop: { value: 1, done: false }
    Loop->>Iterator: next()
    Iterator->>Loop: { value: 2, done: false }
    Loop->>Iterator: next()
    Iterator->>Loop: { value: 3, done: false }
    Loop->>Iterator: next()
    Iterator->>Loop: { value: undefined, done: true }

Topics Covered

Topic	Description	Est. Time
Iteration Protocol	Iterable, iterator, next()	15 min
Custom Iterables	Implementing Symbol.iterator	15 min

Prerequisites

Symbols (Symbol.iterator)
Loops and iteration
Understanding of objects

Learning Path

Iteration Protocol → Custom Iterables