Among all of them, JavaScript is common only on front-end (but you don't really have other choices).
On the back-end, it's used far far less than PHP, C# or Java.
On mobile it's used only as the part of cross-platform development frameworks, and on that field developers are still using Java and Swift more. React Native is a powerful thing, but nothing beats native technologies when it comes to getting best performances of your application.
Regarding the IoT and SmartTV, I can't talk about these two fields but I doubt it has high usage even there (although I think Node could be quite handy for real-time event tracking for example).
Among all of them, JavaScript is common only on front-end (but you don't really have other choices).
On the back-end, it's used far far less than PHP, C# or Java.
On mobile it's used only as the part of cross-platform development frameworks, and on that field developers are still using Java and Swift more. React Native is a powerful thing, but nothing beats native technologies when it comes to getting best performances of your application.
Regarding the IoT and SmartTV, I can't talk about these two fields but I doubt it has high usage even there (although I think Node could be quite handy for real-time event tracking for example).
Absolutely, JavaScript is great, but native is just "greater".
I'm sure the UX is one of the sides JS has been trying to deal with since it started the competition Adobe Flash :D