2 · Build one skill at a time · 18 MIN

Wait for a promise without blocking JavaScript

Await pauses an async flow until a promise settles.

A promise represents a result that is not available yet. Await suspends this module’s continuation while Node can perform other event-loop work. It does not make CPU-heavy JavaScript non-blocking. The timer below simulates waiting; its delay is a minimum rather than an exact schedule. A try/catch around await handles a rejected promise. Real API code should distinguish expected failures from programming errors and should not hide every failure behind an apparently successful result.

Identify where the promise is created. Follow execution up to await, then after settlement. Trigger the deliberate rejection. Restore the code and compare both flows.

Before you start

Install Node.js 22.12+ (or a supported newer LTS). Create a folder named codingneed-node. Open a terminal in that folder. Save ONE lesson example as lesson.mjs and run node lesson.mjs. No npm dependencies are required. HTTP lessons keep running until you press Ctrl+C; stop the previous server before starting another.

File for this example: lesson.mjs

New words, explained

promise: A value representing pending success or failure.
await: Resume this async flow when the promise settles.
rejection: The failure outcome of a promise.

Follow the example step by step

Identify where the promise is created.
Follow execution up to await, then after settlement.
Trigger the deliberate rejection.
Restore the code and compare both flows.

You are ready to move on when: Success prints in the expected order. A rejection reaches the catch block. Explain why a long synchronous loop would still block this process.

Read the example

import {setTimeout as delay} from 'node:timers/promises';
async function loadTopic() {
  await delay(20);
  return 'Node.js';
}
console.log('Starting');
try {
  console.log(await loadTopic());
} catch (error) {
  console.error('Could not load topic:', error.message);
  process.exitCode = 1;
}
console.log('Finished');

Check the expected output

Starting, Node.js and Finished print in that order, each on its own line.

Your challenge

Inside loadTopic, throw new Error("Practice failure") after awaiting the delay. Observe the catch message and nonzero exit code, then restore the success case.

Solution cost: Discuss the operations in this small example; rendering and I/O costs depend on the host. time · Proportional to the example’s retained data. space

Common trap

Calling an async function without awaiting it gives a promise, not its eventual value.

Study the project implementation

import {setTimeout as delay} from 'node:timers/promises';
async function loadTopic() {
  await delay(20);
  return 'Node.js';
}
console.log('Starting');
try {
  console.log(await loadTopic());
} catch (error) {
  console.error('Could not load topic:', error.message);
  process.exitCode = 1;
}
console.log('Finished');

Open guided workshop →See the syllabus