Defer vs Async

⚡ What are Defer and Async?

Defer and Async are HTML script attributes used to optimize the loading of JavaScript files. They allow scripts to load asynchronously or postpone their execution until the page has finished parsing, improving page load times and performance.

⭐ Why are Defer and Async Important for Performance?

Implementing defer and async attributes can significantly enhance a website's performance by reducing render-blocking requests. This is crucial for improving user experience and achieving optimal scores in Core Web Vitals, which are vital for search engine rankings.

⚙️ How Do Defer and Async Work?

1. Scripts with the async attribute load in parallel with other elements and execute as soon as they are available, potentially disrupting DOM construction.
2. Scripts with the defer attribute are loaded in parallel but execute after the document has been parsed, maintaining the order of scripts as they appear in the document.
3. Async is suitable for independent scripts, while defer should be used for scripts relying on DOM elements.

📌 Examples of Using Defer and Async

• Adding async to an analytics script that doesn’t alter the DOM.
• Using defer for a script that manipulates HTML elements or CSS after they have been parsed.

✅ Best Practices for Defer and Async

• Use async for scripts that do not depend on any other scripts or DOM elements.
• Apply defer to scripts that require the DOM to be fully constructed but can be loaded asynchronously.
• Test different configurations to ensure there are no script execution conflicts.

⚠️ Common Mistakes with Defer and Async

• Using async with scripts that modify DOM elements before they are available.
• Not utilizing defer for scripts that should run after HTML parsing is complete.
• Neglecting to test the impact on page load and user interaction.

🛠️ Tools for Analyzing Script Performance

• Lighthouse - audit page loading performance and script execution.
• WebPageTest - detailed view of network requests and script loading.
• GTmetrix - performance testing to analyze page scripts' impact.

📊 Quick Facts About Defer and Async

• Using non-blocking scripts like defer and async can dramatically improve a website’s load speed.
• Many popular websites rely heavily on these attributes for optimized performance.
• Applying these attributes affects the loading sequence of scripts, crucial for maintaining rendering order.

❓ Frequently Asked Questions About Defer and Async

🔍 Related Performance Terms

• Core Web Vitals
• Critical CSS
• DNS Prefetch
• ETag Revalidation

📚 Learn More About Defer and Async

• Web.dev: Core Web Vitals

📝 Key Takeaways

• Defer and Async are used to optimize JavaScript loading in web pages.
• Async loads scripts independently, while defer postpones execution until after parsing.
• Choosing the correct attribute helps improve page performance and user experience.