Script Tag - async & defer
😎 Understanding the Script Tag: async & defer
Hi there! 👋 The script tag is a powerful tool that allows us to include and execute external JavaScript files on our web pages. But have you ever wondered about the attributes async and defer and how they affect the loading and execution of scripts? 🤔 Let's dive into it together and find out! 💪
⚡ Speeding up Site Load
1️⃣ Adding async to the bottom scripts
You mentioned having two external JavaScript files at the bottom of your page. By adding the async attribute to these scripts, you can ensure that they download asynchronously while the rest of the page continues to load. This can potentially speed up your site's overall load time. 🚀
2️⃣ Placing scripts with async in the <head>
If you decide to move these scripts to the <head> section, with the async attribute, they will still download independently of the HTML parsing. However, since they are in the <head> section, they might delay the rendering of the rest of the page until the scripts finish executing. This approach can be useful if your scripts rely heavily on the initial page setup. ⏱️
3️⃣ Impact on different browser versions
HTML4 browsers don't support the async attribute, so adding it to your scripts won't have any effect on their download behavior. Instead, the scripts will be downloaded and executed synchronously. On the other hand, HTML5 browsers will take advantage of the async attribute to download and execute the scripts independently. So, it's important to consider your target audience when deciding between HTML4 and HTML5 compatibility. 🖥️
🔧 Implementing defer
1️⃣ Using defer in the <head>
By adding the defer attribute to your script tags within the <head> section, you ensure that the scripts will be downloaded asynchronously but executed in the order they appear in the HTML. This allows the browser to continue parsing and displaying the content, only executing the scripts after the whole page has been loaded. 🚦
2️⃣ Compatibility with HTML4 browsers
Similar to the async attribute, HTML4 browsers don't support the defer attribute. As a result, the scripts will be downloaded and executed synchronously, potentially impacting your page's load time. It's essential to consider the browser versions your users are using before opting for the defer attribute. 🌐
🚀 Understanding async
If you want your scripts to be downloaded and executed independently without blocking the HTML parsing process, you can use the async attribute. Let's unravel how it works! 🚀
1️⃣ Download behavior
By enabling async for two scripts, they will be downloaded asynchronously, potentially at the same time. However, the order of execution is not guaranteed, as each script will execute as soon as it finishes downloading, regardless of the others. ⏬
2️⃣ Script execution order
If one script depends on the other, and they are both marked as async, there's a possibility that if the first script downloads faster, the second one may not execute correctly. Be cautious when using async for scripts with interdependencies. 🔄
🤔 Should you wait for HTML5?
While it's true that HTML5 browsers provide better support for async and defer, it's crucial to consider your target audience. If a significant portion of your users are still on HTML4 browsers, you might want to weigh the pros and cons before making any changes. It's always a good idea to conduct performance tests and gather user data to make an informed decision. 📊
