Core Async Await Rules
An async function always returns a promise.
await pauses inside async functions until promise settles. Rejections throw errors at await line.
Use try/catch for explicit error handling and predictable user-facing behavior.
JavaScript Tutorial
JavaScript Async Await: From Basics to Production Patterns
async/await makes asynchronous code easier to read and maintain. It is built on promises and is widely used in modern JavaScript.
Why We Need It
Most apps depend on async operations such as API calls and storage reads. Async/await clarity reduces error-prone callback chains.
Syntax
// Add syntax lines here.Basic Example
1. Sequential Flow with Try/Catch
async function loadUserPosts(userId) {
try {
const userRes = await fetch("https://jsonplaceholder.typicode.com/users/" + userId);
const user = await userRes.json();
const postRes = await fetch("https://jsonplaceholder.typicode.com/posts?userId=" + userId);
const posts = await postRes.json();
console.log(user.name, posts.length);
} catch (error) {
...Readable flow where the second request depends on userId.
Real World Example
2. Parallel with Promise.all
async function loadDashboard() {
try {
const [usersRes, postsRes] = await Promise.all([
fetch("https://jsonplaceholder.typicode.com/users"),
fetch("https://jsonplaceholder.typicode.com/posts"),
]);
const [users, posts] = await Promise.all([usersRes.json(), postsRes.json()]);
console.log(users.length, posts.length);
} catch (error) {
console.error("Dashboard error:", error.message);
...Parallel requests reduce total waiting time for independent calls.
Multiple Use Cases
Sequential vs Parallel Execution
Sequential await is readable and best when second task depends on first output.
If tasks are independent, Promise.all should be preferred for parallel speed.
More Examples
3. Safe Fallback Return
async function loadConfig() {
try {
const res = await fetch("/api/config");
if (!res.ok) throw new Error("Config request failed");
return await res.json();
} catch {
return { theme: "light", beta: false };
}
}
...Fallback patterns avoid complete UI breakage.
Comparison
Without
// Without this feature
// ...With
// With this feature
// ...Common Mistakes and Fixes
await outside async function
Wrap logic in async function or supported top-level module context.
Missing try/catch for awaited calls
Use try/catch or handle rejection with .catch.
Sequential awaits for independent tasks
Use Promise.all when tasks do not depend on each other.
Interview Questions
Is async/await better than promises?
It is usually easier to read. Under the hood it still uses promises.
Can I use await inside for loop?
Yes, but execution becomes sequential. Use Promise.all for parallel behavior where safe.
What happens when awaited promise rejects?
An error is thrown at await line and should be handled in try/catch.
Should async functions always return data?
They return promises by default; return explicit data shape for consistency.
Practice Problem
Practice: Write a short example that uses JavaScript Async Await: From Basics to Production Patterns and run it in the compiler.
async function loadUserPosts(userId) {
try {
const userRes = await fetch("https://jsonplaceholder.typicode.com/users/" + userId);
const user = await userRes.json();
const postRes = await fetch("https://jsonplaceholder.typicode.com/posts?userId=" + userId);
const posts = await postRes.json();
console.log(user.name, posts.length);
} catch (error) {
console.error("Load failed:", error.message);
}
}
loadUserPosts(1);Frequently Asked Questions
Is async/await better than promises?
It is usually easier to read. Under the hood it still uses promises.
Can I use await inside for loop?
Yes, but execution becomes sequential. Use Promise.all for parallel behavior where safe.
What happens when awaited promise rejects?
An error is thrown at await line and should be handled in try/catch.
Should async functions always return data?
They return promises by default; return explicit data shape for consistency.
Try It Yourself
Try this example in our JavaScript Compiler and modify it to explore variations.