Difference between Google Analytics script snippets analytics.js and gtag.js

Question

Setting up Google Analytics on one of the websites I have discovered that tracking code snippet provided for embedding in Analytics settings has different syntax and linked JS script than the one, that I have used when setting up another property couple of years ago.

Snippet #1 (gtag.js) - provided by Analytics settings

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

    gtag('config', 'UA-XXXX-Y');
</script>

Snippet #2 (analytics.js) - used in an older website

(function(i,s,o,g,r,a,m){i['GoogleAnalyticsObject']=r;i[r]=i[r]||function(){
(i[r].q=i[r].q||[]).push(arguments)},i[r].l=1*new Date();a=s.createElement(o),
m=s.getElementsByTagName(o)[0];a.async=1;a.src=g;m.parentNode.insertBefore(a,m)
})(window,document,'script','https://www.google-analytics.com/analytics.js','ga');

ga('create', 'UA-XXXX-Y', 'auto');
ga('send', 'pageview');

Documentation refers to migration from analytics.js to gtag.js in general, but without any specific deadlines. So while I am still using analytics.js on my older website the questions arise:

What are the differences (if any) of using gtag.js over analytics.js, both in collected data and in website performance?

Also, when I migrate to gtag.js on older site, are there any known hiccups in collecting data by Google Analytics? Like some data could be lost for some "propagation" period or similar...

Thanks in advance!

Answer 1

gtag.js is not a replacement library for analytics.js, it is a new method of implementation of the analytics.js library. When you look at the javascript that's loaded on a page with gtag.js, you'll see analytics.js being loaded through gtag.js. So data collection is the same.

I would say only change it if there is a need, otherwise, leave as is. analytics.js is fine. I would say gtag.js might be more complicated in terms of implementation compared to analytics.js only because it is new and some people are more used to analytics.js and would confuse gtag() and ga() functions.

Answer 2

Gtag is a javascript library used to send events to various tags like google analytics, floadlight, google Ads etc on same time to reduce the burden of writting different code for different tags for sending data.

On the other hand analytics.js is a js library only used to send data only to google analytics.

Finally I can say that If you want to use only analytics.js in your project then analytics.js is fine but if there is a plan to add any other google services then its better to go with Gtag.js to make things easy.

Note: You can also have a eye on GTM (google tag manager) which will provide more supporting features like third party tags support whereas Gtag only support google services.