Async / Await

TL;DR

async and await are syntactical sugar on top of Promises. They allow you to write asynchronous, non-blocking code that reads like traditional, synchronous code, eliminating .then() chaining.

Mental Model

How It Works

1. async: Placing this keyword before a function guarantees that the function will always return a Promise.
2. await: Can only be used inside an async function. It pauses the execution of that specific function until the Promise settles (resolves or rejects), without blocking the Node.js main thread.

Example

const fs = require('fs').promises; // Use the promise-based version

async function processData() {
    try {
        // Pauses function execution here, but the Event Loop continues!
        const data = await fs.readFile('config.json', 'utf8');
        const parsed = JSON.parse(data);
        console.log('Success:', parsed);
    } catch (err) {
        // Handles both I/O errors and JSON.parse errors
        console.error('Error occurred:', err); 
    }
}

processData();

Common Interview Questions

Does await block the main thread?

No. await only pauses the execution of the async function it is inside. Under the hood, it yields control back to the Node.js Event Loop, allowing other tasks (like handling HTTP requests) to run while waiting.

How do you run multiple await calls in parallel?

If the calls do not depend on each other, do not use await sequentially. Use Promise.all() instead.

// BAD: Takes 2 seconds
const users = await fetchUsers(); 
const posts = await fetchPosts(); 

// GOOD: Takes 1 second
const [users, posts] = await Promise.all([fetchUsers(), fetchPosts()]);