Promise<T>, async/await typing
Understand Promise shapes, annotate async functions, and work with awaited values safely.
Promise<T>, async/await typing is a free TypeScript Academy lesson on CoddyKit. This is lesson 1 of 3. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the TypeScript Academy learning path, and your progress syncs across the web and the CoddyKit app. The TypeScript Academy course includes 3 lessons in total.
Intro
Goal: Read and write Promise<T>-based code. You will type async functions, await values, and keep errors typed.
Promise basics
Promise<T> wraps an eventual value. Use Promise when no value resolves.
function delay(ms: number): Promise<void> {
return new Promise(resolve => setTimeout(resolve, ms));
}
const p: Promise<void> = delay(100);
p.then(() => console.log("done"));async return typing
async functions always return a Promise<U>. The body returns U, which TS wraps automatically.
async function fetchNumber(): Promise<number> {
await delay(10);
return 42; // inferred as Promise<number>
}
fetchNumber().then(n => console.log(n.toFixed(0)));await typing
await unwraps the Promise and yields the inner type (string here), so you get correct IntelliSense.
async function readUserName(id: number): Promise<string> {
await delay(5);
return `user-${id}`;
}
async function main(): Promise<void> {
const name = await readUserName(7); // name: string
console.log(name.toUpperCase());
}
main();Promise.all typing
Promise.all preserves tuple types; each element is awaited to its inner type for you.
async function both() {
const [n, s] = await Promise.all([
fetchNumber(),
readUserName(1),
]);
console.log(n, s);
}
both();Error typing
In catch, prefer unknown. Narrow with instanceof Error (or custom checks) before accessing fields.
async function safe() {
try {
await delay(1);
throw new Error("boom");
} catch (e: unknown) {
if (e instanceof Error) {
console.error("Handled:", e.message);
} else {
console.error("Unknown error", e);
}
}
}
safe();Async return type check
Quick check: What is the return type of an async function that returns a value of type U?
Recap
Recap: Promise<T> wraps eventual values, async functions return Promise<U>, await unwraps types, and errors in catch should start as unknown.
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Frequently Asked Questions
Is the “Promise<T>, async/await typing” lesson free?
Yes — the full text of “Promise<T>, async/await typing” is free to read here on the web. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the TypeScript Academy course, upgrade to CoddyKit PRO. The TypeScript Academy course includes 3 lessons in total.
What will I learn in “Promise<T>, async/await typing”?
Understand Promise shapes, annotate async functions, and work with awaited values safely. You practise TypeScript Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start TypeScript Academy?
No prior experience is required. TypeScript Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 1 of 3.
How long does the “Promise<T>, async/await typing” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this TypeScript Academy lesson?
Yes. Every TypeScript Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- Promise , async/await typing
- Error typing (unknown), narrowing in catch
- Concurrency patterns (all/settled/race) types