Easy

sleep()

Prompt

Write a sleep function in JavaScript that pauses code execution for a given number of milliseconds. The function should return a Promise that resolves after the specified delay.

Playground

Solution

Explanation

What does sleep do?

In many programming languages like Python or Java, there's a built-in way to pause your program for a set amount of time. JavaScript doesn't have this out of the box, but we can build it ourselves using two tools we already have: Promises and setTimeout.

The idea is simple: create a Promise that doesn't resolve right away. Instead, it waits for setTimeout to fire after the given number of milliseconds, and then it resolves.

How the solution works

function sleep(ms) {
  return new Promise((resolve) => setTimeout(resolve, ms));
}

Let's break this single line down:

1. new Promise((resolve) => ...) -- We create a new Promise. The Promise gives us a resolve function that we can call whenever we're "done."
2. setTimeout(resolve, ms) -- We tell JavaScript: "After ms milliseconds, call resolve." That's it. We're just passing resolve directly as the callback to setTimeout.
3. When resolve is called, the Promise transitions from pending to fulfilled, and any code waiting on it (via .then() or await) continues executing.

Walking through the example

console.log('Starting...'); // Runs immediately
await sleep(2000); // Pauses here for 2 seconds
console.log('2 seconds later'); // Runs after the delay

When await sleep(2000) is hit, JavaScript creates a Promise with a 2-second timer. Execution pauses at that line (without blocking the thread). After 2 seconds, setTimeout fires, the Promise resolves, and the next line runs.

Two ways to use it

With .then():

sleep(1000).then(() => {
  console.log('Done!');
});

With async/await:

async function run() {
  await sleep(1000);
  console.log('Done!');
}

Both achieve the same result. The async/await version reads more like synchronous code, which is why most developers prefer it.