async/await in Node.js: writing async code that looks synchronous

async/await Is Syntactic Sugar Over Promises

An async function always returns a Promise. Inside it, await pauses execution of that function (not the event loop) until the awaited Promise resolves. Other code continues running while it waits.

const fs = require('fs').promises;

async function mergeFiles() {
  try {
    const a = await fs.readFile('a.txt', 'utf8');
    const b = await fs.readFile('b.txt', 'utf8');
    await fs.writeFile('out.txt', a + b);
    console.log('Done');
  } catch (err) {
    console.error('Failed:', err.message);
  }
}

mergeFiles();

Sequential vs Parallel

Awaiting inside a loop runs iterations sequentially. Use Promise.all to parallelize:

// Sequential (slow):
for (const file of files) {
  await processFile(file);
}

// Parallel (fast):
await Promise.all(files.map(file => processFile(file)));

Top-Level Await

In ESM files you can use await at the top level without an async wrapper:

// app.mjs
const data = await fs.readFile('config.json', 'utf8');
const config = JSON.parse(data);