What's new to type in TypeScript 5.5+ libs — `Array.fromAsync`, `Set` methods, `Iterator` helpers?

Q: What's new to type in TypeScript 5.5+ libs — `Array.fromAsync`, `Set` methods, `Iterator` helpers?

**Target Audience:** Developers who want to use the newest built-in JavaScript APIs from TypeScript and need to understand where the types come from and what they promise. **Difficulty:** Medium **TypeScript version:** The page runs TypeScript 5.4.5, whose libs have **none** of these three APIs, so the runnable examples use typed userland equivalents. The real lib signatures quoted below were read from, and checked with, **tsc 5.9.3** (`--strict --lib ESNext,DOM`) and **tsc 6.0.2**; that program was then run in **Node 22.22.0**. Release attributions: Set methods in TypeScript 5.5 ([5.5 RC announcement](https://devblogs.microsoft.com/typescript/announcing-typescript-5-5-rc/)), iterator helpers and `IteratorObject` in 5.6 ([5.6 announcement](https://devblogs.microsoft.com/typescript/announcing-typescript-5-6/)). I could not pin down which release first shipped the `Array.fromAsync` declarations: they are absent from 5.4.5 and present in 5.9.3. > **How to read this doc:** Each idea is ex

The problem

Question presented to candidate: "Recent TypeScript releases added types for Set.prototype.union, iterator helpers like values().map().take(), and Array.fromAsync. How do you get those types in a project, what do the signatures look like, and what does TypeScript not do for you here?"

What a strong answer should cover:

Built-in API types come from the lib setting (defaulted from target); new proposals land in esnext.* libs first: Set methods in esnext.collection (TypeScript 5.5), iterator helpers in esnext.iterator (5.6), Array.fromAsync in esnext.array
Types are not polyfills: lib only says the methods exist; the engine (or a polyfill) must actually have them
Set methods: union<U>(other: ReadonlySetLike<U>): Set<T | U>, intersection gives Set<T & U>, difference keeps Set<T>; "set-like" means size, has, keys (a Map qualifies, an array does not)
Iterator helpers are lazy and return IteratorObject<T, undefined, unknown>; 5.6 introduced IteratorObject, ArrayIterator and friends plus BuiltinIteratorReturn and the --strictBuiltinIteratorReturn strict flag
Array.fromAsync returns Promise<T[]>, awaiting each element in order (unlike Promise.all)
Version detail: in TypeScript 6.0 the Set and iterator types moved to an es2025 lib, while fromAsync stays in esnext

Code / implementation expected: Optional — a tsconfig lib change plus a few typed calls, or a typed userland equivalent.

esnext
lib-types

Try it yourself

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Solution

Target Audience: Developers who want to use the newest built-in JavaScript APIs from TypeScript and need to understand where the types come from and what they promise. Difficulty: Medium TypeScript version: The page runs TypeScript 5.4.5, whose libs have none of these three APIs, so the runnable examples use typed userland equivalents. The real lib signatures quoted below were read from, and checked with, tsc 5.9.3 (--strict --lib ESNext,DOM) and tsc 6.0.2; that program was then run in Node 22.22.0. Release attributions: Set methods in TypeScript 5.5 (5.5 RC announcement), iterator helpers and IteratorObject in 5.6 (5.6 announcement). I could not pin down which release first shipped the Array.fromAsync declara

The solution is hidden so you can try first

14 min read · 10 sections

Run the code

Edit the code and run it here. Each console line with an expected value becomes a test.

TypeScriptSet operations with the lib signatures (union, intersection, difference)