On the server-side, you can detect the browser making the request and determine whether to bother sending the polyfill script tag in the response.

If you are concerned about modern browsers unnecessarily downloading and running the polyfill all for naught, then there are techniques you can to do minimize the impact: From my experience with polyfills, I have not noticed any significant performance implications. Since most of the modern browsers already support many of the native ES6 API features, many of your users will get the full performance of the browser's native implementation. With a polyfill, only the older browsers may have slower performance compared to the native implementation. The polyfill is only needed when you want to use an ES6 feature AND you need to support older browsers. Many of the new native API features can be used today by providing a polyfill. When the day comes that when all the browsers your site supports have support for all the ES6 features, then you can turn off the transpiling step in your build and you'll get the full benefit of native browser performance without having to manually convert your manually written ES5 code to ES6.

Also, writing your code with ES6 is shorter and easier to maintain. The code generated by babel has been thoroughly tested by babel's test suite and has been used by thousands of developers around the world. It also beats me trying to write equivalent ES5 code manually. The ES5 equivalent code that is generated has performed just fine. Until all the browsers your site supports have support for the new ES6 language features, you won't be able to use them in your scripts without first transpiling them to an ES5 equivalent. Keep in mind that there are two main parts to ES6: Checkout the ES6 compatibility table to determine what features you can use today for the browsers your site supports. This means you have to keep an eye on all indention, as any extra space/tab will be captured into the string.Until more browsers support ES2015 (aka ES6) features, using transpilers and polyfills will be the only way to go.

Back-tick strings are verbatim and may be multi-line.

Es6 features