Delay, Sleep, Pause & Wait in JavaScript

JavaScript lacks built-in sleeping functions, but don't worry! This article will explore various ways to implement latency in JavaScript code while keeping the language's asynchronous features in mind.

Key Points

1. Implementing Delay in Asynchronous JavaScript: This article explores various techniques for implementing Delay in JavaScript code, emphasizing the asynchronous nature of the language. Unlike many other programming languages ​​with built-in sleep functions, JavaScript requires alternative methods to introduce delays, which this article aims to explain and demonstrate.
2. Understand the execution model of JavaScript: The key aspect of implementing latency in JavaScript is understanding its execution model. JavaScript handles asynchronous operations differently than languages ​​such as Ruby, which can lead to unexpected behavior in timing and operation sequence. This article will highlight this difference through examples, showing how the event-driven model of JavaScript affects the execution of delayed functions.
3. Best Practice for Creating Delays: This article describes several ways to create delays, such as using setTimeout, Promise, and async/await, and discusses the best use cases for each method. It also explains common pitfalls such as blocking event loops and provides effective solutions and best practices to effectively manage time and asynchronous operations in JavaScript development.

How to create a sleeping function in JavaScript

For readers who just want to solve problems quickly without digging into technical details, here is the most direct way:

<code class="language-javascript">function sleep(ms) {
  return new Promise(resolve => setTimeout(resolve, ms));
}

console.log('Hello');
sleep(2000).then(() => { console.log('World!'); });</code>

Run this code and you will see "Hello" pop up in the console. Then, after a brief pause of two seconds, "World!" will follow. This is a neat and effective way to introduce delays without any effort.

It would be great if you came just for this! But if you are curious about "why" and "how", there is more to learn. There are some nuances and complexities when dealing with time in JavaScript that you may find useful. So keep reading to learn more!

Understand the execution model of JavaScript

Now that we have mastered a quick solution, let's dive into the mechanisms of JavaScript execution models. Understanding this is essential to effectively manage time and asynchronous operations in your code.

Consider the following Ruby code:

<code class="language-ruby">require 'net/http'
require 'json'

url = 'https://api.github.com/users/jameshibbard'
uri = URI(url)
response = JSON.parse(Net::HTTP.get(uri))
puts response['public_repos']
puts 'Hello!'</code>

As one would expect, this code makes a request to the GitHub API to get my user data. It then parses the response, outputs the number of public repositories that belong to my GitHub account, and finally prints "Hello!" to the screen. The execution order is from top to bottom.

Compare this with the equivalent JavaScript version:

<code class="language-javascript">function sleep(ms) {
  return new Promise(resolve => setTimeout(resolve, ms));
}

console.log('Hello');
sleep(2000).then(() => { console.log('World!'); });</code>

If you run this code, it will first output "Hello!" to the screen, and then output the number of public repositories that belong to my GitHub account.

This is because getting data from the API is an asynchronous operation in JavaScript. The JavaScript interpreter will encounter the fetch command and schedule the request. However, it does not wait for the request to complete. Instead, it will continue to execute, outputting "Hello!" to the console, and then when the request returns after a few hundred milliseconds, it will output the number of repositories.

If these contents are new information to you, you should watch this excellent conference speech: What the heck is the event loop anyway?

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How to use SetTimeout in JavaScript correctly

Now that we have a better understanding of JavaScript's execution model, let's take a look at how JavaScript handles latency and asynchronous code.

The standard way to create delays in JavaScript is to use its setTimeout method. For example:

<code class="language-ruby">require 'net/http'
require 'json'

url = 'https://api.github.com/users/jameshibbard'
uri = URI(url)
response = JSON.parse(Net::HTTP.get(uri))
puts response['public_repos']
puts 'Hello!'</code>

This will record "Hello" to the console and then record "World!" in two seconds. In many cases, this is enough: perform some operations, and then perform others after a brief delay. Get it done!

But unfortunately, things are not always that simple.

You might think setTimeout will pause the entire program, but that is not the case. It is an asynchronous function, which means the rest of your code won't wait for it to finish.

For example, suppose you run the following code:

<code class="language-javascript">fetch('https://api.github.com/users/jameshibbard')
  .then(res => res.json())
  .then(json => console.log(json.public_repos));
console.log('Hello!');</code>

You will see the following output:

<code class="language-javascript">console.log('Hello');
setTimeout(() => {  console.log('World!'); }, 2000);</code>

Note how "Goodbye!" appears before "World!"? This is because setTimeout does not prevent the execution of the remaining code.

This means you can't do this:

<code class="language-javascript">console.log('Hello');
setTimeout(() => { console.log('World!'); }, 2000);
console.log('Goodbye!');</code>

"Hello" and "World" will be logged to the console immediately without any noticeable delay.

You can't do this either:

<code>Hello
Goodbye!
World!</code>

Seep a second to think about what might happen in the above code snippet.

It will not print numbers 0 to 4 with one second delay. Instead, the numbers 0 to 4 will be recorded to the console at the same time after a one-second delay. Why? Because the loop will not pause execution. It won't wait for to complete before proceeding to the next iteration. setTimeout

To get the desired output, we need to adjust the delay in

to setTimeout milliseconds. i * 1000

<code class="language-javascript">console.log('Hello');
setTimeout(1000);
console.log('World');</code>

This staggers the execution of the

statement, ensuring that there is a one-second interval between each output. console.log

The key here is that

setTimeout will not block the execution of the program, but the JavaScript interpreter will continue to process the rest of the code and will return to the execution callback function only after the timer expires. So what is the use of

? Let's take a look now.

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