Programming Fundamentals › Concurrency & Async

async / await

Syntax for writing asynchronous code that reads like synchronous code.

Also known as: async await, async functions, async/await

async and await let you write code that waits for slow things (network, disk, timers) in a straight line, instead of chaining callbacks.

async function loadUser(id) {
  const res = await fetch(`/api/users/${id}`);   // pause here until the response arrives
  const user = await res.json();
  return user;                                   // the function returns a promise
}

Rules:

• await works inside an async function (and at the top level of ES modules).
• An async function always returns a promise.
• While one function waits, the program can run other work. It is not blocking the whole app (see blocking vs non-blocking).

Python has the same shape with asyncio:

import asyncio

async def load_user(user_id):
    await asyncio.sleep(0.1)          # stand-in for a network call
    return {"id": user_id}

asyncio.run(load_user(1))

Errors use normal try/catch

try {
  const user = await loadUser(7);
} catch (err) {
  showError(err);
}

Common mistakes

1. Forgetting await. You get a promise (JavaScript) or a coroutine that never ran (Python warns “coroutine … was never awaited”) instead of the value.

2. Awaiting things one after another that are independent:

const a = await getA();       // waits...
const b = await getB();       // ...then starts this. Slow.

const [a, b] = await Promise.all([getA(), getB()]);   // both at once

3. await in forEach. items.forEach(async (x) => await save(x)) doesn’t wait for anything. Use a for...of loop, or Promise.all with map.