What are defer and async attributes in script tags?

In modern web development, scripts play a crucial role in enhancing the functionality and interactivity of a web page. However, scripts can also slow down the loading and rendering of a page. To optimize the performance of web pages and improve user experience, the HTML <script> tag provides two important attributes: defer and async. These attributes control how scripts are loaded and executed in a web page.

Key Takeaways

The defer attribute delays the execution of a script until after the HTML document has been parsed. Scripts with the defer attribute are downloaded and executed in the order they appear in the document, while allowing the browser to continue parsing and building the DOM.
The async attribute, on the other hand, allows the script to be executed as soon as it is available. Scripts with the async attribute are downloaded and executed independently of other scripts and the DOM construction.
The defer attribute is suitable for scripts that rely on the DOM or rely on the execution order of other scripts, while the async attribute is preferred for independent scripts that can run in any order.
Both defer and async attributes are optional. If neither attribute is present, or if the <script> tag doesn’t have a src attribute, the script will be fetched and executed synchronously, blocking the parsing of the HTML document.

What do defer and async mean?

The defer and async attributes in the HTML <script> tag are used to control the loading and execution of scripts in a web page. The defer attribute allows a script to be downloaded in the background while the HTML document is parsed, and then executed after the document has been fully parsed. The async attribute, on the other hand, allows a script to be downloaded and executed independently of other scripts and the DOM construction. The choice between defer and async depends on the script’s dependency on the DOM and the desired execution order.

A Step-by-Step Guide to Using defer and async Attributes

To effectively utilize the defer and async attributes in the <script> tag, follow these steps:

Identify the scripts in your web page that can be loaded asynchronously or deferred to improve performance.
Determine the script’s dependency on the DOM and its desired execution order. If the script relies on the availability of the DOM or the execution order of other scripts, consider using the defer attribute. If the script is independent and the execution order doesn’t matter, consider using the async attribute.
Add the defer or async attribute to the <script> tag:

For deferred script loading, add the defer attribute:

<script src="script.js" defer></script>

For async script loading, add the async attribute:

<script src="script.js" async></script>

Save the changes and test your web page in different browsers to ensure compatibility.

Who are defer and async Attributes For?

The defer and async attributes in the HTML <script> tag are for web developers and designers who want to optimize the loading and performance of their web pages. These attributes can be particularly useful in scenarios where scripts need to be loaded and executed without blocking the rendering of the page, or when the order of script execution is crucial.

Web developers who have a good understanding of how scripts can impact performance and have control over script execution order will benefit the most from using defer and async attributes.

Now that we understand the basics of defer and async attributes, let’s dive deeper into their behaviors and implications.

Understanding the defer Attribute

The defer attribute allows a script to be executed after the HTML document has been fully parsed. Scripts with the defer attribute are downloaded in the background while the browser continues parsing and building the DOM.

It is important to note that the defer attribute only applies to external scripts that have a src attribute. Inline scripts and scripts without a src attribute are always executed synchronously and are not affected by the defer attribute.

The Order of Script Execution with defer

Scripts with the defer attribute are executed in the order they appear in the document. This means that if you have multiple scripts with the defer attribute, they will be executed in the same order in which they are defined in the HTML.

Consider the following example:

<script src="script1.js" defer></script>
<script src="script2.js" defer></script>

In this case, script1.js will be executed before script2.js because it appears first in the document.

Deferring Scripts and DOM Dependency

One of the primary use cases for the defer attribute is when a script depends on the availability of the DOM. By deferring the script execution, you ensure that the DOM has been fully constructed before the script runs.

For example, if your script manipulates or interacts with elements on the page using the document.getElementById() method, it is important to ensure that the DOM is ready before executing the script. The defer attribute can help achieve this.

Browser Support for defer

The defer attribute is supported in all modern browsers and IE9+. However, it is important to note that the behavior of defer can vary across browsers, particularly when it comes to the execution order of deferred scripts.

In some browsers, like Chrome and Firefox, the relative order of deferred scripts is maintained, ensuring that scripts are executed in the order they appear in the document. However, in other browsers, like Safari, the execution order may not be preserved, leading to potential issues if scripts have dependencies on each other.

It is recommended to test your web pages with deferred scripts in multiple browsers to ensure consistent behavior.

Understanding the async Attribute

The async attribute allows scripts to be downloaded and executed asynchronously, without blocking the parsing and rendering of the HTML document. Scripts with the async attribute are executed as soon as they are available, regardless of whether the DOM has been fully constructed.

Like the defer attribute, the async attribute only applies to external scripts with a src attribute.

The Order of Script Execution with async

Unlike the defer attribute, scripts with the async attribute do not preserve their relative order of execution. This means that if you have multiple scripts with the async attribute, they can be executed in any order as soon as they are available.

For example:

<script src="script1.js" async></script>
<script src="script2.js" async></script>

In this case, script1.js and script2.js can be executed in any order, depending on their download and availability. This behavior allows scripts to be loaded and executed independently, improving parallelism and potentially reducing the overall loading time.

Implications for Scripts with Dependencies

The async attribute is best suited for scripts that are independent and do not have dependencies on other scripts or the DOM. If your scripts have dependencies or require the DOM to be fully constructed, it is recommended to use the defer attribute instead.

When using the async attribute, it is important to ensure that scripts do not rely on each other or the availability of the DOM. If scripts have dependencies, it is possible for them to be executed before the required dependencies are available, leading to unexpected errors or issues.

Browser Support for async

The async attribute is supported in all modern browsers and IE10+. However, similar to the defer attribute, the behavior of async can vary across browsers, particularly when it comes to the execution order of asynchronously loaded scripts.

It is important to test your web pages with asynchronously loaded scripts in multiple browsers to ensure consistent behavior.

Conclusion

In this article, we have explored the defer and async attributes in the HTML <script> tag and their impact on script loading and execution in a web page. By using these attributes, web developers can improve the performance of their web pages by controlling the loading behavior of scripts.

The defer attribute allows scripts to be executed after the HTML document has been parsed, preserving their relative order of execution and allowing the browser to continue parsing and building the DOM. This attribute is especially useful when scripts have dependencies on the DOM or other scripts.

The async attribute, on the other hand, enables scripts to be loaded and executed independently of other scripts and the DOM construction. This attribute is suitable for scripts that are independent and do not have dependencies on the DOM or other scripts. However, it is important to consider potential issues when scripts have dependencies or require the DOM to be fully constructed.

To ensure cross-browser compatibility and consistent behavior, it is recommended to test web pages with deferred and asynchronously loaded scripts in various browsers.

By understanding and utilizing the defer and async attributes effectively, web developers can optimize the loading and execution of scripts, resulting in faster and more efficient web pages.

Remember to regularly test and optimize your scripts for optimal performance to provide the best possible user experience.

Now that you have a better understanding of the defer and async attributes in the <script> tag, go ahead and apply this knowledge to your own web development projects!