creating-iterators.md

Creating synchronous iterators

The methods of class Iterator let us process data incrementally. Let’s explore where we can use them.

Getting iterators from iterables

• Iterator.from(iterable) always returns instances of Iterator (converting non-instances to instances as needed).
• iterable[Symbol.iterator]() returns an iterator:
  • With all built-in data structures, the result is an instance of Iterator.
  • With other, older iterable objects, the result may not be an instance of Iterator.

Built-in methods that return iterators

Arrays, Typed Arrays, Sets and Maps have additional methods that return iterators:

• Arrays (similarly: Typed Arrays):
  • Array.prototype.keys() returns an iterator over numbers.
  • Array.prototype.values() returns an iterator.
  • Array.prototype.entries() returns an iterator over key-value pairs. The keys are numbers.
• Sets:
  • Set.prototype.values() returns an iterator.
  • Set.prototype.keys() returns an iterator. Equivalent to .values().
  • Set.prototype.entries() returns an iterator over value-value pairs (i.e., both components of the pair are the same value).
• Maps:
  • Map.prototype.keys() returns an iterator.
  • Map.prototype.values() returns an iterator.
  • Map.prototype.entries() returns an iterator over key-value pairs.

The following methods return iterators:

• String.prototype.matchAll() returns an iterator over match objects.

Other sources of iterators

Generators also return iterators:

function* gen() {}

assert.equal(
  gen() instanceof Iterator,
  true
);