When developing a website, it's important to consider how scripts will affect the loading performance of the page. Using the "async" and "defer" attributes on the <script> tag can help improve the loading speed and overall user experience.
In HTML, the <script> tag is used to embed or reference a script (typically written in JavaScript) within an HTML document. However, there are two additional attributes that can be added to the <script> tag to affect how the script is loaded and executed: async and defer.
Async
The "async" attribute tells the browser to download the script while the page continues to load and render. Once the script has finished downloading, it is executed immediately, without blocking the browser's rendering process. The "async" attribute is particularly useful for scripts that are not critical to the initial loading or rendering of the page. For example, a script that tracks user analytics or loads a third-party advertisement can be loaded asynchronously without affecting the user's ability to interact with the page. This means that these scripts will be downloaded and executed in the background while the user can continue to interact with the page.
Defer
The "defer" attribute tells the browser to download the script while the page continues to load and render, but to hold off on executing the script until the page has finished loading. For example, if a script uses the jQuery library, which is also included in the page, it should use the defer attribute to ensure that jQuery is loaded and ready to use before the script is executed. This helps to avoid any potential errors or unexpected behavior.
It's also important to note that while "async" and "defer" both allow the browser to download scripts while the page is loading, they behave differently when it comes to execution.
With "async" attribute, the browser will execute the script as soon as it is downloaded, which can cause problems if the script relies on other resources that have not yet loaded. With "defer", the script will only execute after the page has finished loading, so there's no risk of the script executing prematurely.
In general, if you're unsure whether to use "async" or "defer", start by using "defer" and only use "async" if necessary for performance reasons.
In conclusion, it's crucial to understand the difference between <script>, <script async> and <script defer> when developing a website. The "async" attribute allows for the browser to execute the script as soon as it's downloaded and does not block the rendering of the page, while "defer" attribute allows the browser to download the script while the page is loading and execute after the page is finished loading. This can help improve the loading speed and overall user experience of the website.