rauschma · June 3, 2025 16:33 · Jun 3, 2025
diff --git a/creating-iterators.md b/creating-iterators.md
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+# Creating synchronous iterators
+
+[The methods of class `Iterator`](https://2ality.com/2022/12/iterator-helpers.html) 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:
+
+```js
+function* gen() {}
+
+assert.equal(
+  gen() instanceof Iterator,
+  true
+);
+```
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