Typing Promises

Promise<T> tells TypeScript what a successful asynchronous computation resolves to. A Promise<string> can reject for many reasons, but TypeScript does not encode the rejection type in Promise<T>; catch values should be handled carefully.

Reading the code: loadScore returns Promise<number>. Await extracts its fulfilled number in main. main itself returns Promise<void>, because an async function always produces a Promise.

Keep the boundary clear: Typing Promise<number> does not guarantee the operation will succeed; rejection is still possible.

Save the code as typing-promises.ts. The result below was captured after compiling this exact example with strict TypeScript checking and executing the emitted JavaScript in Node.js.

TypeScript code

function loadScore(): Promise<number> { return Promise.resolve(88); }
async function main(): Promise<void> {
  const score: number = await loadScore();
  console.log(`score=${score}`);
}
main().catch(console.error);

Verified output

score=88

Build: npx tsc --strict --target es2022 --module commonjs --lib es2022,dom,esnext.disposable typing-promises.ts; run: node typing-promises.js. The exact filename may be changed freely. When a multi-file project exists, prefer its tsconfig.json and build scripts.

loadScore returns Promise<number>. Await extracts its fulfilled number in main. main itself returns Promise<void>, because an async function always produces a Promise.

The output shown above comes from the initialized values in this program. Replacing a type annotation alone does not transform a JavaScript value.

Typing Promise<number> does not guarantee the operation will succeed; rejection is still possible.

Check both the TypeScript diagnostic and the emitted JavaScript behavior. A program can compile correctly and still need runtime validation or error handling.

Change the function to Promise<{ score:number }> and update the consumer to read score.

Before running your change, write down the type you expect TypeScript to infer and the output you expect JavaScript to print. Then check both predictions.

Promise<T> tells TypeScript what a successful asynchronous computation resolves to.

Try the practice change and re-run the exact program to test your understanding.