Detect if browser has smooth scrolling feature already

I managed to solve it this way:

<style>
  body {
    scroll-behavior: smooth;
  }
</style>

<script>
 if(getComputedStyle(document.body).scrollBehavior === 'smooth'){
   console.log('This browser supports scrollBehavior smooth');
 }
</script>

Yes and no. Unfortunately there are no standards for these types of things. However there are work arounds, one of which you are already doing: browser sniffing.

Basically, that's about as advanced as this kind of detection goes because some browsers don't yet even support smooth scrolling officially or without experimental developments (like Chromium). And standards won't be set unless the majority are on the same page. Not only that but it also comes down to GPU hardware abilities as some devices and computers struggle with smooth scrolling and animations. So technically speaking, even if a browser did support smooth scrolling, who's to say the device or desktop running it can render fast enough to even display the effect. That's another bottle neck.

I believe someday in the future there will be more of a need for browser feature specifications such as this to better improve user interactions, but until that day you're doing it right.