Adding Deferred Loading To WordPress Plugins’ Javascript

Why Deferred JavaScript Matters

Implementing deferred JavaScript loading techniques in WordPress plugins can significantly improve website performance and user experience. By loading scripts asynchronously and preventing them from blocking initial page rendering, pages load faster and feel more responsive to users.

Defining JavaScript files as deferred allows the HTML document to be parsed and displayed to users without having to wait for all scripts to be fetched and executed. This reduces the initial page load time, allowing above-the-fold content to render more quickly. Studies show that 47% of users expect a webpage to load in 2 seconds or less, so optimizing load speed is critical.

Slower page loads frustrate users, hurting engagement metrics. Deferred scripts allow webpages to begin rendering content in as little as 200 milliseconds during initial loads, down from 1 second or more when blocking scripts are used. This improved performance reduces bounce rates and boosts pages-per-session.

How Deferred JavaScript Works

Deferred scripts are loaded asynchronously in the background without blocking the HTML document parsing. This allows the browser to continue constructing the DOM, apply styles, and render page content even before deferred scripts have completely downloaded or executed.

When deferred scripts are ready to be used, they execute after the window’s load event fires. This signals that core HTML content has been parsed and basic rendering completed. Defining the loading behavior this way prevents deferred scripts from manipulating DOM elements before they exist, reducing errors.

By carefully scripting deferral strategies, web developers can optimize the delivery of meaningful page content first. Non-essential scripts that produce below-the-fold content, enable analytics, create interactive widgets and more can be queued asynchronously so as not to compete for resources.

Implementing Deferred JavaScript in Plugins

WordPress developers can easily add deferred loading attributes to registered scripts in plugins. This is accomplished using the wp_script_add_data function.

For example, when enqueuing JavaScript files, the $in_footer parameter can be set to true. This will automatically move the script to the footer and enable async loading by the browser:

wp_register_script( 'my-script', 'path/to/my-script.js', array(), '1.0.0', true ); 

wp_enqueue_script( 'my-script' );

No other changes are necessary. WordPress handles converting the script over to deferred status when it is rendered on the front-end. The async attribute will keep it from blocking page rendering.

Other techniques like listening for the window load or DOMContentLoaded events may be used within JavaScript code itself. Wrapping code in functions that fire after the document is fully built can ensure elements targeted by scripts are accessible.

document.addEventListener("DOMContentLoaded", function() {
  // Javascript code goes here  
});

Debugging and Troubleshooting

Although deferred scripts improve performance, potential issues may arise during implementation:

• Scripts that depend on jQuery may fail if jQuery is not loaded first. Check script load order.
• Some browser extensions conflict with async/deferred scripts. Test across browsers.
• Scripts that need to edit above-the-fold elements will not execute quickly enough. Load these render-blocking.
• Errors may arise if scripts access DOM elements before fully loaded. Wrap logic in document ready functions.

Test plugins on a variety of devices and browsers to uncover and address subtle issues that only present under specific conditions. Modern browsers will timeout scripts after a few seconds, avoiding crashes but review error logs.

Use debugging tools like browser developer consoles to identify conflicts. The Network tab tracks loading times of individual files requests to help tune performance.

Best Practices For Deferred JavaScript

To maximize results when adding deferred attributes to scripts, adhere to these guidelines:

• Defer third-party scripts whenever possible to avoid blocking. Exception is made for feature libraries like jQuery.
• Load functionality critical scripts like WordPress admin tools without deferral.
• Place deferred scripts in page footer using $in_footer parameter.
• Adhere to best practices within JavaScript code to support async loading.
• Simplify logic and break into functions to minimize dependencies.
• Test across browsers and fix individual quirks as needed.

Carefully planned deferred loading strategies allow WordPress plugins to boost webpage performance dramatically while creating smooth user experiences. By converting plugins’ JavaScript files over to asynchronous approaches, render-blocking is reduced and initial loads optimized for conversions.