I "know" javascript in the same way that I "know" SQL, or JVM assembly. It's part of the stack, you do want to be able to read and understand it, but you don't use it for programming (that is, expressing business logic) in.
Different senses of "stack". It's inevitable that there will be multiple technologies involved in the running of your code, which will involve multiple programming languages, and you will need to understand the behaviour of those technologies (though less and less as they mature - I wouldn't be surprised if it were possible to debug a compile-to-js language without understanding JS these days). But that's not the same thing as being able to program in those languages.
“It's inevitable you will have multiple languages / platforms comprising your stack.”