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 thedefer
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 theasync
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 theasync
attribute is preferred for independent scripts that can run in any order. - Both
defer
andasync
attributes are optional. If neither attribute is present, or if the<script>
tag doesn't have asrc
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 theasync
attribute. - Add the
defer
orasync
attribute to the<script>
tag:- For deferred script loading, add the
defer
attribute: - For async script loading, add the
async
attribute:
- For deferred script loading, add the
- 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!