How do you reverse a string in-place in JavaScript?

As long as you’re dealing with simple ASCII characters, and you’re happy to use built-in functions, this will work:

function reverse(s){
    return s.split("").reverse().join("");
}

If you need a solution that supports UTF-16 or other multi-byte characters, be aware that this function will give invalid unicode strings, or valid strings that look funny. You might want to consider this answer instead.

[…s] is Unicode aware, a small edit gives:-

function reverse(s){
    return [...s].reverse().join("");
}