Defer: Delayed but Organized Execution
- Background Loading: When you use the
- DOM-Friendly: One of the primary advantages of
deferis that it doesn’t block the DOM construction. The script is executed only after the entire DOM has been built, which ensures a smooth user experience.
- Execution Timing: Although
deferscripts load asynchronously, they maintain their original order of execution as listed in the HTML document. These scripts are executed in the order they appear in the DOM.
- DOM Load: The DOM load event waits until all deferred scripts are downloaded and executed, ensuring that the webpage is fully functional before scripts are run.
- External Scripts: The
deferattribute is typically applied to external scripts and helps improve the performance of pages containing large or complex scripts.
Async: Independent and Immediate Execution
async attribute introduces a different loading strategy that offers unique benefits for specific scenarios:
- Independence: Unlike
asyncloads scripts in parallel with other page elements, but it does not wait for other scripts or the DOM to be fully loaded before executing. This makes it particularly useful for third-party scripts not tightly coupled to your site’s functionality.
- Immediate Execution: Scripts marked with
asyncare executed immediately after downloading, even if the DOM is not yet ready. This can lead to potential issues if the script relies on DOM elements that haven’t been constructed yet.
- Third-Party Integration:
asyncis an excellent choice for integrating third-party components such as ads, analytics, and counters. These elements often operate independently from the rest of your page’s functionality.
Choosing the Right Approach
async offer benefits. It’s crucial to select the appropriate strategy based on your website’s requirements:
- Use Async: Integrating self-contained third-party scripts that do not require a fully loaded DOM. This is great for scenarios where scripts don’t need to wait for one another or for the DOM to be ready.
defer depends on the nature of your scripts and your specific performance goals. By understanding the differences and considering the benefits and downsides of each approach, you can ensure that your website loads efficiently and delivers a seamless user experience.