defer in Javascript at Five Levels

Let’s talk about the concept of “defer” in the context of JavaScript and HTML:

1. To a Child: Imagine you’re getting ready for school, and your mom tells you to put on your shoes, but not until after you’ve put on your clothes. The ‘defer’ in JavaScript is like your mom telling you to wait to put on your shoes. It tells the browser to wait to run a script until after the rest of the page has loaded.

2. To a Teenager: You know when you’re watching a video online, and you can keep browsing the site while the video loads? That’s kind of like what ‘defer’ does in JavaScript. When you add ‘defer’ to a script tag, it tells the browser not to run the script right away, but to continue building the website and then run the script after it’s finished loading the page.

3. To an Undergraduate Computer Science Major: The ‘defer’ attribute in JavaScript is used in a script tag to specify that the script should not execute until the entire HTML document has been fully parsed. Without ‘defer’, scripts are fetched and executed immediately when they’re encountered, which can block the parsing of the rest of the HTML document and delay the time to interactive.

4. To a Graduate Computer Science Student: ‘defer’ is an attribute that can be added to a script tag in HTML. When present, it instructs the browser to defer the execution of the JavaScript file until the HTML parser has completed. This approach improves the loading performance of web pages by ensuring that script execution doesn’t interfere with the initial page render. However, it’s important to note that ‘defer’ only works with external scripts (<script src="...">) and not inline scripts.

5. To a Colleague (Professional Developer): The ‘defer’ attribute is used in a script tag to indicate to the browser that the execution of the script should be deferred until the HTML document is completely parsed. When a script tag has the ‘defer’ attribute, the script is fetched asynchronously while the HTML document is parsing but is executed only after the document has been fully parsed. This makes ‘defer’ an essential tool in optimizing page load performance because it prevents JavaScript from blocking the rendering of the webpage. It’s important to note, however, that ‘defer’ scripts execute in the order they appear in the document and they execute before DOMContentLoaded is fired.