---
source: https://iinventedlight.com/library/notes/typescript-iterators-generators.md
title: "Iterators and generators in TypeScript"
license: CC-BY-4.0
license_url: https://creativecommons.org/licenses/by/4.0/
source_license: "CC BY 4.0 (microsoft/TypeScript-Website); prose restates cited source lines"
attribution: "Summary by iinventedlight (AI-assisted). Sources: TypeScript (Microsoft), TypeScript: Documentation - Iterators and Generators, CC BY 4.0 (microsoft/TypeScript-Website); TypeScript (Microsoft), TypeScript: TSConfig Option: downlevelIteration, CC BY 4.0 (microsoft/TypeScript-Website)."
author: "iinventedlight (synthesis of cited sources); text drafted with AI assistance"
date: 2026-11-04
status: published
checked_on: 2026-11-04
as_of: 2026-11-04
confidence: medium
untested: "No code was run for this note; only the fetched documentation pages were read."
category: coding-references
topic_area: "code-ai-eng: TypeScript"
tags: ['typescript', 'iterators', 'generators', 'es2015']
sources:
  - id: S1
    title: "TypeScript: Documentation - Iterators and Generators"
    publisher: TypeScript (Microsoft)
    url: https://www.typescriptlang.org/docs/handbook/iterators-and-generators.html
    accessed: 2026-11-04
    license: "CC BY 4.0 (microsoft/TypeScript-Website)"
  - id: S2
    title: "TypeScript: TSConfig Option: downlevelIteration"
    publisher: TypeScript (Microsoft)
    url: https://www.typescriptlang.org/tsconfig/downlevelIteration.html
    accessed: 2026-11-04
    license: "CC BY 4.0 (microsoft/TypeScript-Website)"
---
# Iterators and generators in TypeScript

**Answer:** An object is iterable in TypeScript (and JavaScript) if it implements `Symbol.iterator`. Arrays, Maps, Sets and strings already do. When targeting ES5, enable `downlevelIteration` for accurate iteration semantics.

## for..of vs for..in

```ts
const list = [4, 5, 6];
for (const i in list) console.log(i); // "0","1","2"
for (const v of list) console.log(v); // 4,5,6
```

## A generator

```ts
function* range(n: number): Generator<number> {
  for (let i = 0; i < n; i++) yield i;
}
for (const i of range(3)) console.log(i);
```

## Custom iterable

```ts
class Bag<T> implements Iterable<T> {
  constructor(private items: T[]) {}
  *[Symbol.iterator]() { yield* this.items; }
}
```

## Targeting ES5

ES2015 added `for..of`, array spread, argument spread and `Symbol.iterator`.

## Claims
- id: C1
  claim: An object is iterable if it implements Symbol.iterator.
  source: S1
  evidence: "An object is deemed iterable if it has an implementation for the Symbol.iterator property."
- id: C2
  claim: Built-in types like Array, Map and Set implement Symbol.iterator.
  source: S1
  evidence: "Some built-in types like Array, Map, Set, String, Int32Array, Uint32Array, etc. have their"
- id: C3
  claim: ECMAScript 6 added new iteration primitives like for/of.
  source: S2
  evidence: "ECMAScript 6 added several new iteration primitives: the for / of loop"
- id: C4
  claim: downlevelIteration enables accurate iteration in ES5 environments.
  source: S2
  evidence: "downlevelIteration allows for these iteration primitives to be used more accurately in ES5 environments"

## Sources
- [S1] TypeScript: Documentation - Iterators and Generators. TypeScript (Microsoft). https://www.typescriptlang.org/docs/handbook/iterators-and-generators.html . License: CC BY 4.0 (microsoft/TypeScript-Website). Accessed 2026-11-04.
- [S2] TypeScript: TSConfig Option: downlevelIteration. TypeScript (Microsoft). https://www.typescriptlang.org/tsconfig/downlevelIteration.html . License: CC BY 4.0 (microsoft/TypeScript-Website). Accessed 2026-11-04.

License: this note is CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Summary by iinventedlight (AI-assisted).