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Practice Problems

What is async/await in JavaScript

async and await are syntactic sugar over promises, introduced in ES2017, which allows writing asynchronous code as if it were synchronous.

  • async — makes function asynchronous, automatically returning Promise.
  • await — pauses execution inside async function until promise completes (successfully or with error).

Simple Example

javascript
function delay(ms) { return new Promise(resolve => setTimeout(resolve, ms)); } async function greet() { console.log("Waiting..."); await delay(1000); // Wait 1 second console.log("Hello after 1 second!"); } greet();

async: How Does it Work?

javascript
async function foo() { return 42; } foo().then(result => console.log(result)); // 42

Even if function returns simple value, async wraps it in Promise.

await: How Does it Work?

javascript
async function fetchUser() { const res = await fetch("/user.json"); const data = await res.json(); return data; }
  • await pauses execution until fetch returns result.
  • After completion, result is passed to variable and execution continues.

Error Handling

With async/await it's convenient to use try/catch:

javascript
async function getData() { try { const response = await fetch("/api"); const data = await response.json(); console.log(data); } catch (error) { console.error("An error occurred:", error); } }

Features

  • await can only be used inside async function.
  • await only works with promises (or any thenable objects).
  • If await returns error, it's thrown and caught in catch.

Summary

  • async/await makes asynchronous code understandable, linear and clean.
  • It doesn't replace promises, but simplifies their use.
  • Always handle errors with try/catch or .catch().

Good to Know:

await doesn't block main thread! It only pauses current async function execution, freeing thread for other tasks.

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