Alfie Kemp

Alfie Kemp


Delay, Sleep, Pause, & Wait in JavaScript

Many programming languages have a sleep function that will delay a program’s execution for a given number of seconds. This functionality is absent from JavaScript, however, owing to its asynchronous nature. In this article, we’ll look briefly at why this might be, then how we can implement a sleep function ourselves.

We’ve rewritten this popular guide from scratch to provide the best and most up-to-date advice. This article was updated in November 2019.

Understanding JavaScript’s Execution Model

Before we get going, it’s important to make sure we understand JavaScript’s execution model correctly.

Consider the following Ruby code:

require 'net/http'
require 'json'

url = ''
uri = URI(url)
response = JSON.parse(Net::HTTP.get(uri))
puts response['public_repos']
puts "Hello!"

As one might expect, this code makes a request to the GitHub API to fetch my user data. It then parses the response, outputs the number of public repos attributed to my GitHub account and finally prints “Hello!” to the screen. Execution goes from top to bottom.

Contrast that with the equivalent JavaScript version:

  .then(res => res.json())
  .then(json => console.log(json.public_repos));

If you run this code, it will output “Hello!” to the screen, then the number of public repos attributed to my GitHub account.

This is because fetching data from an API is an asynchronous operation in JavaScript. The JavaScript interpreter will encounter the fetch command and dispatch the request. It will not, however, wait for the request to complete. Rather, it will continue on its way, output “Hello!” to the console, then when the request returns a couple of hundred milliseconds later, it will output the number of repos.

If any of this is news to you, you should watch this excellent conference talk: What the heck is the event loop anyway?.

You Might Not Actually Need a Sleep Function

Now that we have a better understanding of JavaScript’s execution model, let’s have a look at how JavaScript handles delays and asynchronous operations.

Create a Simple Delay Using setTimeout

The standard way of creating a delay in JavaScript is to use its setTimeout method. For example:

setTimeout(() => {  console.log("World!"); }, 2000);

This would log “Hello” to the console, then after two seconds “World!” And in many cases, this is enough: do something, wait, then do something else. Sorted!

However, please be aware that setTimeout is an asynchronous method. Try altering the previous code like so:

setTimeout(() => { console.log("World!"); }, 2000);

It will log:


Waiting for Things with setTimeout

It’s also possible to use setTimeout (or its cousin setInterval) to keep JavaScript waiting until a condition is met. For example, here’s how you might use setTimeout to wait for a certain element to appear on a web page:

function pollDOM () {
  const el = document.querySelector('my-element');

  if (el.length) {
    // Do something with el
  } else {
    setTimeout(pollDOM, 300); // try again in 300 milliseconds


This assumes the element will turn up at some point. If you’re not sure that’s the case, you’ll need to look at canceling the timer (using clearTimeout or clearInterval).

If you’d like to find out more about JavaScript’s setTimeout method, please consult our tutorial which has plenty of examples to get you going.

Flow Control in Modern JavaScript

It’s often the case when writing JavaScript that we need to wait for something to happen (for example, data to be fetched from an API), then do something in response (such as update the UI to display the data).

The example above uses an anonymous callback function for this purpose, but if you need to wait for multiple things to happen, the syntax quickly gets pretty gnarly and you end up in callback hell.

Luckily, the language has evolved considerably over the past few years and now offers us new constructs to avoid this.

For example, using async await we can rewrite the initial code to fetch information from the GitHub API:

(async () => {
  const res = await fetch(``);
  const json = await res.json();

Now the code executes from top to bottom. The JavaScript interpreter waits for the network request to complete and the number of public repos is logged first, then the “Hello!” message.

If this is more the sort of thing you’re trying to accomplish, I encourage you to read our article Flow Control in Modern JS: Callbacks to Promises to Async/Await.

Bringing Sleep to Native JavaScript

If you’re still with me, then I guess you’re pretty set on blocking that execution thread and making JavaScript wait it out.

Here’s how you might do that:

function sleep(milliseconds) {
  const date =;
  let currentDate = null;
  do {
    currentDate =;
  } while (currentDate - date < milliseconds);


As expected, this will log “Hello”, pause for two seconds, then log “World!”

It works by using the method to get the number of milliseconds that have elapsed since January 1, 1970 and assigning that value to a date variable. It then creates an empty currentDate variable, before entering a do ... while loop. In the loop it repeatedly gets the number of milliseconds which have elapsed since January 1, 1970 and assigns the value to the previously declared currentDate variable. The loop will keep going while the difference between date and currentDate is less than the desired delay in milliseconds.

Job done, right? Well not quite …

A Better Sleep Function

Maybe this code does exactly what you’re hoping for, but be aware, it has a large disadvantage: the loop will block JavaScript’s execution thread and ensure that nobody can interact with your program until it finishes. If you need a large delay, there’s a chance that it may even crash things altogether.

So what to do?

Well, it’s also possible to combine the techniques learned earlier in the article to make a less intrusive sleep function:

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

sleep(2000).then(() => { console.log("World!"); });

This code will log “Hello”, wait for two seconds, then log “World!” Under the hood we’re using the setTimeout method to resolve a Promise after a given number of milliseconds.

Notice that we need to use a then callback to make sure the second message is logged with a delay. We can also chain more callbacks onto the first:

  .then(() => { console.log("World!"); })
  .then(() => {
      .then(() => { console.log("Goodbye!"); })

This works, but looks ugly. We can pretty it up using async ... await:

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

async function delayedGreeting() {
  await sleep(2000);
  await sleep(2000);


This looks nicer, but means that whatever code is using the sleep function needs to be marked as async.

Click here.

Of course, both of these methods still have the disadvantage (or feature) that they do not pause the entire program execution. Only your function sleeps:

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

async function delayedGreeting() {
  await sleep(2000);


The code above logs the following:



Timing issues in JavaScript are the cause of many a developer headache, and how you deal with them depends on what you’re trying to achieve.

Although a sleep function is present in many other languages, I’d encourage you to embrace JavaScript’s asynchronous nature and try not to fight the language. It’s actually quite nice when you get used to it.

#javascript #programming

What is GEEK

Buddha Community

Delay, Sleep, Pause, & Wait in JavaScript

Rahul Jangid


What is JavaScript - Stackfindover - Blog

Who invented JavaScript, how it works, as we have given information about Programming language in our previous article ( What is PHP ), but today we will talk about what is JavaScript, why JavaScript is used The Answers to all such questions and much other information about JavaScript, you are going to get here today. Hope this information will work for you.

Who invented JavaScript?

JavaScript language was invented by Brendan Eich in 1995. JavaScript is inspired by Java Programming Language. The first name of JavaScript was Mocha which was named by Marc Andreessen, Marc Andreessen is the founder of Netscape and in the same year Mocha was renamed LiveScript, and later in December 1995, it was renamed JavaScript which is still in trend.

What is JavaScript?

JavaScript is a client-side scripting language used with HTML (Hypertext Markup Language). JavaScript is an Interpreted / Oriented language called JS in programming language JavaScript code can be run on any normal web browser. To run the code of JavaScript, we have to enable JavaScript of Web Browser. But some web browsers already have JavaScript enabled.

Today almost all websites are using it as web technology, mind is that there is maximum scope in JavaScript in the coming time, so if you want to become a programmer, then you can be very beneficial to learn JavaScript.

JavaScript Hello World Program

In JavaScript, ‘document.write‘ is used to represent a string on a browser.

<script type="text/javascript">
	document.write("Hello World!");

How to comment JavaScript code?

  • For single line comment in JavaScript we have to use // (double slashes)
  • For multiple line comments we have to use / * – – * /
<script type="text/javascript">

//single line comment

/* document.write("Hello"); */


Advantages and Disadvantages of JavaScript

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The technology used to develop the overall app by the developers from WebClues Infotech is at par with the latest available technology.

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Niraj Kafle


The essential JavaScript concepts that you should understand

As a JavaScript developer of any level, you need to understand its foundational concepts and some of the new ideas that help us developing code. In this article, we are going to review 16 basic concepts. So without further ado, let’s get to it.

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Ajay Kapoor


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Santosh J


JavaScript compound assignment operators

JavaScript is unarguablly one of the most common things you’ll learn when you start programming for the web. Here’s a small post on JavaScript compound assignment operators and how we use them.

The compound assignment operators consist of a binary operator and the simple assignment operator.

The binary operators, work with two operands. For example a+b where + is the operator and the a, b are operands. Simple assignment operator is used to assign values to a variable(s).

It’s quite common to modify values stored in variables. To make this process a little quicker, we use compound assignment operators.

They are:

  • +=
  • -+
  • *=
  • /=

You can also check my video tutorial compound assignment operators.

Let’s consider an example. Suppose price = 5 and we want to add ten more to it.

var price = 5;
price = price + 10;

We added ten to price. Look at the repetitive price variable. We could easily use a compound += to reduce this. We do this instead.

price += 5;

Awesome. Isn’t it? What’s the value of price now? Practice and comment below. If you don’t know how to practice check these lessons.

Lets bring down the price by 5 again and display it.
We use console.log command to display what is stored in the variable. It is very help for debugging.
Debugging let’s you find errors or bugs in your code. More on this later.

price -= 5;

Lets multiply price and show it.

price *=5;

and finally we will divide it.

price /=5;

If you have any doubts, comment below.

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