Async return type

2 snippets in TypeScript

TSTypeScript

Return Type Annotations

syntax

function fn(): ReturnType { ... }
const fn = (): ReturnType => { ... };

example

function findUser(id: string): User | undefined {
  return userDatabase.get(id);
}

// Arrow function return type
const formatCurrency = (amount: number): string => {
  return `$${amount.toFixed(2)}`;
};

// Async function
async function fetchOrder(id: string): Promise<Order> {
  const resp = await fetch(`/api/orders/${id}`);
  return resp.json();
}

output

// Async functions always return Promise<T>

Note Explicit return types are especially valuable for public API functions, async functions, and functions with multiple return paths. They also speed up type-checking in large projects since the compiler does not need to analyze the function body.

Awaited<T>

TS · Utility Types

syntax

type Result = Awaited<PromiseType>;

example

type PromisedUser = Promise<{ id: string; name: string }>;
type ResolvedUser = Awaited<PromisedUser>;
// { id: string; name: string }

// Handles nested promises
type DeepPromise = Promise<Promise<Promise<number>>>;
type DeepResolved = Awaited<DeepPromise>;
// number

// Practical: extract resolved type from async function
async function loadConfig() {
  return { apiUrl: "https://api.example.com", timeout: 5000 };
}
type Config = Awaited<ReturnType<typeof loadConfig>>;

output

// Awaited recursively unwraps all layers of Promise

Note Added in TS 4.5. Awaited recursively unwraps nested Promises until it hits a non-Promise type. Before Awaited existed, developers had to write custom recursive unwrap types. Works with any thenable, not just native Promises.