Async vs Defer

Introduction

When it comes to website performance, loading time is a crucial factor that affects user experience. JavaScript plays a vital role in the interactivity and functionality of a web page, but it can also slow down a website by blocking the rendering process. Fortunately, there are two attributes that can help optimize the loading time of JavaScript files - 'async' and 'defer'.

Async

The async attribute is used to load JavaScript files asynchronously, meaning that the file will be downloaded and executed while the rest of the page continues to load. This is useful for non-essential scripts such as tracking codes or ads because they don't block the rendering process. However, the order of execution may not be guaranteed, which could cause issues with dependencies.

Here's an example of how to use the async attribute:

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

Defer

The defer attribute is also used to load JavaScript files asynchronously, but with one key difference - the script will be executed in the order they are defined in the HTML. This is useful for scripts that have dependencies on other scripts or the DOM because it ensures that the necessary resources are available.

Here's an example of how to use the defer attribute:

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

Conclusion

In summary, the 'async' attribute is useful for non-essential scripts that don't have dependencies, while the 'defer' attribute is useful for scripts that have dependencies and need to be executed in order. By using these attributes correctly, you can optimize the loading time of your website and provide a better user experience.