How to Delay JavaScript to Boost Page Speed

In recent years, approaching users on multiple channels such as advertising websites, social networks, or live chat services has become more and more popular. And then, using scripts to insert those services into a website is the most important technique. However, using too many 3rd party scripts will cause a slower loading speed of the website and its Page Speed scores may be worse. At that time, you often have just 2 options: accept the slow loading or remove some scripts. After a while of researching, we found a way to have one more option. It’s delaying the execution or loading of JavaScript. Let's see how!

Table of Contents hide

1. Why delaying JavaScript?
2. How to delay JavaScript in WordPress
3. Should (not) you delay scripts?
4. Last words

Why delaying JavaScript?

Data downloaded from third-party servers like Facebook Page Widget, Facebook Messenger, Facebook Comments, iframe or live chat services like Tawk.to are data that you cannot control. You cannot compress, merge or cache them, simply because they are not on your host. These data are often very heavy and can cause serious problems related to website loading speed. To see this clearly, you can use Google PageSpeed Insights, GTmetrix, or any other speed test tools to verify.

And since you cannot optimize them, the only solution to integrate the above services into the website without affecting the page speed is to delay the execution of their scripts. In this way, you will reduce your page render time and improve speed indexes on page speed testing tools such as Time to Interactive, First CPU Idle, Max Potential Input Delay, etc. This will also reduce the initial payload on the browser by reducing the number of requests.

Note: This technique is based on the code of Flying Scripts, which allows you to delay scripts in WordPress. The plugin has more control options in the admin area where you don't need to touch code. So, check it out if you want that. Otherwise, let's discover how the code works below.

How to delay JavaScript in WordPress

The delay script

To delay scripts, we need to write a custom code. This code is called "delay script" and its job is delaying other scripts. In other words, it's used to load other scripts on the website.

You can put this script in the <head> or <body> tag. But you should put it in the <head> tag to run it at the same time with the lazy load scripts. It is more reasonable for most cases. If the delayed scripts are in the <head> tag, they will not work when placed in the <body> tag because the script will be loaded just after the whole page finished loading.

<script>
{
    const load = () => {
        document.querySelectorAll("script[data-type='lazy']").forEach(el => el.src = el.dataset.src);
        document.querySelectorAll("iframe[data-type='lazy']").forEach(el => el.src = el.dataset.src);
    };
    const timer = setTimeout(load, 5000);
    const trigger = () => {
        load();
        clearTimeout(timer);
    };
    ["mouseover","keydown","touchmove","touchstart"].forEach(e => window.addEventListener(e, trigger, {passive: true, once: true}));
}
</script>

This is the minified version of that:

<script>{const e=()=>{document.querySelectorAll("script[data-type='lazy']").forEach((e=>e.src=e.dataset.src)),document.querySelectorAll("iframe[data-type='lazy']").forEach((e=>e.src=e.dataset.src))},t=setTimeout(e,5e3),a=()=>{e(),clearTimeout(t)};["mouseover","keydown","touchmove","touchstart"].forEach((e=>window.addEventListener(e,a,{passive:!0,once:!0})))}</script>

The script above will delay both JavaScript and iframes. If you want to delay only JavaScript, use this instead:

<script>
{
    const load = () => document.querySelectorAll("script[data-type='lazy']").forEach(el => el.src = el.dataset.src);
    const timer = setTimeout(load, 5000);
    const trigger = () => {
        load();
        clearTimeout(timer);
    };
    ["mouseover","keydown","touchmove","touchstart"].forEach(e => window.addEventListener(e, trigger, {passive: true, once: true}));
}
</script>

This is the minified version of that:

<script>{const e=()=>document.querySelectorAll("script[data-type='lazy']").forEach((e=>e.src=e.dataset.src)),t=setTimeout(e,5e3),o=()=>{e(),clearTimeout(t)};["mouseover","keydown","touchmove","touchstart"].forEach((e=>window.addEventListener(e,o,{passive:!0,once:!0})))}</script>

Note: You can use the Slim SEO plugin to to add code to the header. This plugin allows you to insert any code to the header, body, or footer. So you can use it to insert Webmaster tool verification codes or tracking scripts.

How does the script work?

The script will detects if there's any user interaction on the page (moving mouse or press any keys, buttons or clicks), and then look for all the delayed scripts or iframes and load them. Because there will still be scripts you want to execute right away, the above script doesn’t delay the execution of all the scripts on your site. In there, we specified that only scripts with the attribute data-type='lazy' (you can rename the attribute freely) will be delayed.

In case there are no interactions at all on the script, it will load all delayed scripts after 5 seconds. This way, all the scripts are always loaded regardless the user interactions.

And because the loading process happens only when there is at least one user interaction, those delayed scripts won't be loaded when the page is just loaded, which makes it load very fast. And if you measure the loading speed, or the Google Pagespeed score, you'll see both of them are great.

Modify existing scripts

After adding the delay script, we need to modify exising scripts on your website to make them delayed. For each type of script, there will be a different way to do that. Here is how to do it for some popular scripts like Google Tag Manager, Facebook Customer Chat, Youtube, or Google Maps.

Google Tag Manager

Below is the default Google Tag Manager script:

<script>(function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':
new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],
j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src=
'https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);
})(window,document,'script','dataLayer','YOUR-GTM-ID');</script>

You can rewrite it with the delay script as follows:

<script>
window.dataLayer = window.dataLayer || [];
window.dataLayer.push({'gtm.start': new Date().getTime(), event: 'gtm.js'});
</script>
<script data-type="lazy" data-src="https://www.googletagmanager.com/gtm.js?id=YOUR-GTM-ID"></script>

Google Analytics

The default Google Analytics code looks like this:

<script async src="https://www.googletagmanager.com/gtag/js?id=YOUR-ID"></script>
<script>
window.dataLayer = window.dataLayer || [];
function gtag(){dataLayer.push(arguments);}
gtag('js', new Date());

gtag('config', 'YOUR-ID');
</script>

To delay it, change it to:

<script>
window.dataLayer = window.dataLayer || [];
function gtag(){dataLayer.push(arguments);}
gtag('js', new Date());
gtag('config', 'YOUR-ID');
</script>
<script data-type="lazy" data-src="https://www.googletagmanager.com/gtag/js?id=YOUR-ID"></script>

Facebook Customer Chat

Here is the default Facebook script, used to load the Customer Chat widget:

<div id="fb-root"></div>
<div id="fb-customer-chat" class="fb-customerchat"></div>
<script>
    var chatbox = document.getElementById('fb-customer-chat');
    chatbox.setAttribute("page_id", "YOUR_PAGE_ID");
    chatbox.setAttribute("attribution", "biz_inbox");

    window.fbAsyncInit = function() {
        FB.init({
            xfbml : true,
            version : 'v12.0'
        });
    };

    (function(d, s, id) {
        var js, fjs = d.getElementsByTagName(s)[0];
        if (d.getElementById(id)) return;
        js = d.createElement(s); js.id = id;
        js.src = 'https://connect.facebook.net/en_US/sdk/xfbml.customerchat.js';
        fjs.parentNode.insertBefore(js, fjs);
    }(document, 'script', 'facebook-jssdk'));
</script>

In it, the code is in the paragraph (function()...); used to load chat widgets to your website.

I will shorten it by removing the function() code and adding a script like this right below the the code as follows:

<script data-type='lazy' data-src="https://connect.facebook.net/en_US/sdk/xfbml.customerchat.js"></script>

Then, the code of Facebook Customer Chat will look like this:

<div id="fb-root"></div>
<div id="fb-customer-chat" class="fb-customerchat"></div>
<script>
    var chatbox = document.getElementById('fb-customer-chat');
    chatbox.setAttribute("page_id", "YOUR_PAGE_ID");
    chatbox.setAttribute("attribution", "biz_inbox");

    window.fbAsyncInit = function() {
        FB.init({
            xfbml : true,
            version : 'v12.0'
        });
    };
</script>
<script data-type='lazy' data-src="https://connect.facebook.net/en_US/sdk/xfbml.customerchat.js"></script>

If you use another plugin of Facebook such as Facebook Comment, Facebook Widget, or other live chat services such as Tawk.to, you can do likewise.

Youtube

Youtube and Google Maps use iFrame tags. With this tag, you just have to add data-type='lazy' and data-src like this:

The default of Youtube is:

<iframe src="https://www.youtube.com/embed/I3ncHxLxwlM"></iframe>

Now, I will change it into this:

<iframe data-type='lazy' data-src="https://www.youtube.com/embed/I3ncHxLxwlM"></iframe>

Google Maps

The default iFrame of Google Maps is:

<iframe src="https://www.google.com/maps/embed/v1/place?key=API_KEY&q=Space+Needle,Seattle+WA"></iframe>

So I will change it by adding data- type='lazy' inside the tag <iframe> like this:

<iframe data-type='lazy' data-src="https://www.google.com/maps/embed/v1/place?key=API_KEY&q=Space+Needle,Seattle+WA"></iframe>

Other Scripts

If you want to apply this method to the common scripts, you just need to replace src into data-src and add the data-type='lazy' attribute.

An example is:

<script src="custom-javascript.js"></script>

Then, change it into this:

<script data-src="custom-javascript.js" data-type='lazy'></script>

Should (not) you delay scripts?

This technique should be used for scripts related to the user interaction or live chat like Facebook Customer Chat, Facebook Widget, Facebook Comment, iframe (Youtube, Google Maps), Tawk.to, …

Otherwise, it isn’t recommended for scripts like tracking or analyzing user data such as Google Analytics, Facebook Pixel, Google Tag Manager, Crazy Egg, Google Remarketing Tag, ... Because applying this technique may cause recording data incompletely or inaccurately. You certainly won't want to miss this data, right?

However, even in the case you use tracking scripts, you still want to delay them, to increase the performance of the website. That will give you a better pagespeed score, better Core Web Vitals and as they're a ranking factor, it might help you rank better on Google.

Last words

Delaying the script execution method will help you optimize your website’s loading and increase page speed scores as well. There are many ways to increase the speed of websites; so, consider what is more suitable for you to use. If you cannot apply this method to your websites, you can read about other methods in this series.

If you have any questions, feel free to let us know in the comment section. Good luck!

3 thoughts on “How to Delay JavaScript to Boost Page Speed”

Kiran More says:

April 5, 2022 at 3:11 PM

can we refractor this code for livechat widget

1. Cuong Tran says:
  
  April 5, 2022 at 5:16 PM
  
  This technique can be solved with live chat widgets such as Facebook Customer Chat, Facebook Widget, Facebook Comment, iframe (Youtube, Google Maps), Tawk.to
Mica Kaka says:

October 4, 2022 at 6:13 AM

Hello, so how setimeout after user interaction ? I mine for exemple, after user click or other interaction on the page how to add timeout before script execution ?

Thanks.