Understanding and Implementing async and defer in JavaScript

When working with JavaScript in HTML, especially when dealing with external scripts, understanding the difference between async and defer becomes crucial. This article aims to clarify when to use each attribute and how they impact your website's performance and user experience.

What are the Default and async Attributes?

The simplest form of the script tag in HTML, without any attributes, ensures that the HTML file is parsed until the script tag is encountered. At this point, the browser pauses the parsing of the HTML file to fetch the script from an external source and execute it. This can significantly slow down page rendering, especially if the script is heavy or complex.

Understanding async and defer Attributes

To better manage script loading and improve page performance, HTML offers two important attributes: async and defer. Here’s a detailed look at each:

async

The async attribute tells the browser to download the script while the HTML is being parsed but to execute it as soon as it's downloaded, without waiting for the download to complete. While this can improve parsing and parsing pause times, if the script takes a long time to download, the browser will continue to pause the parsing, which can still affect page rendering.

defer

The defer attribute behaves similarly to async in that it also downloads the script during HTML parsing. However, it does not execute the script immediately but waits until the HTML parsing is complete. This ensures that the script runs in the order it appears in the document, which can be useful for scripts that depend on each other to function correctly.

When Should You Use async and defer?

To optimize your website, you should follow these general guidelines for using async, defer, or no attributes:

Using async

Use the async attribute for modules of JavaScript that do not depend on each other and can be executed independently. This includes scripts that do not rely on any other scripts to function properly. By setting async, you allow the browser to load these scripts concurrently with the HTML parsing, which can enhance page load times.

Using defer

Use the defer attribute if your script relies on or is dependent on the scripts that precede it, or if using async would violate the intended execution order. Since defer scripts execute only after the HTML parsing is complete, it ensures that the scripts are executed in the order they appear in the document. This is particularly useful when dealing with complex scripts that build upon each other.

No Attribute (Default Behavior)

If your script is critical and needs to be executed as soon as possible, or if it relies on other scripts that you want to execute in a specific order, use the default behavior with no attribute. This ensures that the script is loaded and executed before the rest of the page content is rendered, potentially blocking the rendering process.

Best Practices and Examples

Here are some examples to illustrate the application of these attributes and best practices:

Example: Using async for Independent Scripts

script srcpath/to/independentScript.js async/script

In this example, the external script is loaded and executed as soon as it is downloaded, which can improve page load times if the script is modular and does not depend on other scripts.

Example: Using defer for Dependent Scripts

script srcpath/to/dependentScript.js defer/script

This script will be loaded during HTML parsing but will be executed only after the HTML parsing is complete, ensuring that it runs in the correct sequence relative to other scripts in the document.

Example: Using Inline Scripts with No Attributes

script  // Small script that is relied upon by an async script  function essentialFunction() {    // Essential code  }/script

This inline script is executed as soon as the browser encounters it. If this script is relied upon by an async script, it should be placed above the async script to maintain the correct execution order.

Conclusion

Properly managing script loading can greatly enhance the performance and user experience of your website. By understanding and utilizing the async and defer attributes effectively, you can optimize your scripts to load and execute in a way that improves page load times and maintains the intended execution order.

Related Keywords and Terms

Keywords: async, defer, HTML5, JavaScript, script loading

Terms: HTML script tag, modular scripts, dependency injection, parser-blocking, non-blocking, parsing pause times