Many developers often wonder about the timing of deferred scripts execution concerning the `DOMContentLoaded` event. Understanding this can help optimize the performance of your web applications. Let's delve into this topic and clarify how deferred scripts are handled in relation to the `DOMContentLoaded` event.
When you include a script in your HTML document, it's essential to consider when and how that script gets executed. Scripts can be included in two main ways: synchronously and asynchronously. However, there's also a third way called deferred scripts.
Deferred scripts are parsed by the browser along with the rest of the HTML content but don't get executed immediately. Instead, they are executed after the parsing is complete. This can have significant implications for the loading and rendering of your web page.
Now, the `DOMContentLoaded` event is fired when the initial HTML document has been completely loaded and parsed, without waiting for stylesheets, images, and subframes to finish loading. The question arises: Are deferred scripts executed before the `DOMContentLoaded` event?
The answer is yes. Deferred scripts are indeed executed before the `DOMContentLoaded` event is triggered. This behavior ensures that deferred scripts are given a chance to execute before the DOM content is fully loaded.
This sequencing can be advantageous in scenarios where you have scripts that rely on the DOM being in a certain state to function correctly. By using deferred scripts, you ensure that these scripts run before the `DOMContentLoaded` event fires, allowing them access to the necessary DOM elements.
To make a script deferred in your HTML, you can use the `defer` attribute within the script tag. For example:
By including the `defer` attribute, you inform the browser that this script should be executed after parsing is complete but before the `DOMContentLoaded` event.
It's important to note that the order of deferred script execution is preserved as they appear in the document. If you have multiple deferred scripts, they will be executed sequentially in the order they are defined in the HTML file.
Understanding these nuances of script execution timing can help you optimize the loading and performance of your web applications. By leveraging deferred scripts strategically, you can ensure that critical scripts are executed at the right moment in the page loading process.
In conclusion, deferred scripts are executed before the `DOMContentLoaded` event, making them a valuable tool for managing script execution timing in your web projects. Experiment with deferred scripts in your own code to see how they can improve the efficiency and user experience of your web applications.