I was going to say this and scopes, but since it's mentioned already, I will say: Compilers and Presets and Browser Support!
There is so many to figure it out, TypeScript & Babel & ES2015 & ES2016 & ES2017 . and the whole ECMAScript 6 Compatibility with browsers, See This Compatibility Table for example.
If a client asks to support IE8 I just make him pay its support as a brand new project. That's it.
We do exactly the same, old and non standard browsers are quoted as an extra and separate part of the project.
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