Today I’m going to discuss a new, and very nice, site speed improvement that became possible in WordPress 4.1. What changed? The introduction of a new filter, script_loader_tag. This filter lets us easily change the HTML markup of enqueued script elements—that is, of JavaScript files that were correctly added into a WordPress site using WordPress’s wp_enqueue_script function.

With script_loader_tag, we can now easily fix a problem that can significantly impact page speed: lots of render-blocking JavaScript.

The Problem: Render-Blocking JavaScript

Long JavaScript files in your head can delay your browser from displaying page content, because its default behavior is first to interpret the JS files themselves.

Properly enqueued JavaScript shows up in the head section of your HTML document. On the internet as in nature, the main thing about a head is that it’s above a body—and this means something fairly serious for site speed, because JavaScript can be render-blocking.

“Render-blocking” comes from a web browser’s default behavior: It wants to completely receive and process everything that’s come higher up in the page, before it moves any further down.

This means that long JavaScript files in your head can actually delay your browser from displaying the page content in the body, because its default behavior is first to interpret the JS files themselves. In other words, JS is blocking the browser’s crucial function of rendering the page out for the user to actually see. The result can be slow sites and frustrated users.

.. The Fix: Defer and Async your JavaScript

The first thing to understand is the alternatives to render-blocking JS: defer and async. We’ll explain the difference, but both work similarly: They let the browser load a JS resource “as time permits,” while attending to other things (like page rendering) as well. This means that you can’t rely on a deferred or asynced JavaScript file being in place prior to page render, as you could without these attributes—but the advantage is that the file won’t slow the speed at which the page becomes visible to users.

Those are concepts—now for code. (The full code is available on GitHub.)

 

.. 2. Deferring and Asyncing Render-Blocking JavaScript

We found that we needed to use defer and not async for WPShout, so I’ll walk through the defer code. Most of the heavy lifting here is from an article by Scott Nelle; thanks, Scott!

<span class="token function">add_filter<span class="token punctuation">(</span></span> <span class="token string">'script_loader_tag'</span><span class="token punctuation">,</span> <span class="token string">'wsds_defer_scripts'</span><span class="token punctuation">,</span> <span class="token number">10</span><span class="token punctuation">,</span> <span class="token number">3</span> <span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">function</span> <span class="token function">wsds_defer_scripts<span class="token punctuation">(</span></span> <span class="token variable">$tag</span><span class="token punctuation">,</span> <span class="token variable">$handle</span><span class="token punctuation">,</span> <span class="token variable">$src</span> <span class="token punctuation">)</span> <span class="token punctuation">{</span>

<span class="token comment" spellcheck="true">	// The handles of the enqueued scripts we want to defer
</span>	<span class="token variable">$defer_scripts</span> <span class="token operator">=</span> <span class="token keyword">array</span><span class="token punctuation">(</span> 
		<span class="token string">'prismjs'</span><span class="token punctuation">,</span>
		<span class="token string">'admin-bar'</span><span class="token punctuation">,</span>
		<span class="token string">'et_monarch-ouibounce'</span><span class="token punctuation">,</span>
		<span class="token string">'et_monarch-custom-js'</span><span class="token punctuation">,</span>
		<span class="token string">'wpshout-js-cookie-demo'</span><span class="token punctuation">,</span>
		<span class="token string">'cookie'</span><span class="token punctuation">,</span>
		<span class="token string">'wpshout-no-broken-image'</span><span class="token punctuation">,</span>
		<span class="token string">'goodbye-captcha-public-script'</span><span class="token punctuation">,</span>
		<span class="token string">'devicepx'</span><span class="token punctuation">,</span>
		<span class="token string">'search-box-value'</span><span class="token punctuation">,</span>
		<span class="token string">'page-min-height'</span><span class="token punctuation">,</span>
		<span class="token string">'kamn-js-widget-easy-twitter-feed-widget'</span><span class="token punctuation">,</span>
		<span class="token string">'__ytprefs__'</span><span class="token punctuation">,</span>
		<span class="token string">'__ytprefsfitvids__'</span><span class="token punctuation">,</span>
		<span class="token string">'jquery-migrate'</span><span class="token punctuation">,</span>
		<span class="token string">'icegram'</span><span class="token punctuation">,</span>
		<span class="token string">'disqus'</span><span class="token punctuation">,</span>
	<span class="token punctuation">)</span><span class="token punctuation">;</span>

    <span class="token keyword">if</span> <span class="token punctuation">(</span> <span class="token function">in_array<span class="token punctuation">(</span></span> <span class="token variable">$handle</span><span class="token punctuation">,</span> <span class="token variable">$defer_scripts</span> <span class="token punctuation">)</span> <span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">return</span> <span class="token string">'<script src="'</span> <span class="token punctuation">.</span> <span class="token variable">$src</span> <span class="token punctuation">.</span> <span class="token string">'" defer="defer" type="text/javascript"></script>'</span> <span class="token punctuation">.</span> <span class="token string">"\n"</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
    
    <span class="token keyword">return</span> <span class="token variable">$tag</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span> 

The add_filter line tells us that this code should run anytime an enqueued JavaScript file is about to be printed onto the page as an HTML script element. Letting us filter that HTML is what script_loader_tag is for. (If you need an update on filters and WordPress’s hooks system in general, start here!)

 

GitHub project: