How can I implement a sleep function in JavaScript?

JavaScript doesn't have a built-in sleep function, which is probably a good idea considering how much trouble it could cause if used incorrectly. The closest equivalent is the setTimeout() function, but there are other, less common ways to implement a function that will pause execution for a specified amount of time.

Using setTimeout()

JavaScript's setTimeout() sets a timer which executes some code once the timer expires. Only the code inside the setTimeout() callback will execute after the timer expires. This can lead to nesting issues, as well as code executing out of order if you are not careful.

const delay = (fn, ms, ...args) => setTimeout(fn, ms, ...args);

const greet = (name) => console.log(`Hello ${name}!`);
delay(greet, 300, 'world');
// Logs: Hello world! (after 300ms)

Asynchronous version, using promises

Another way to go about implementing a sleep() function is to utilize the async and await keywords, a Promise and setTimeout(). Note that the resulting function is itself asynchronous.

const sleep = (ms) =>
  new Promise(resolve => setTimeout(resolve, ms));

const greet = async () => {
  console.log('I will be with you in just a moment.');
  await sleep(300);
  console.log('Hello there!');
};
greet();
// Logs: I will be with you in just a moment.
// Logs: Hello there! (after 300ms)