consider the following
➜ ~ node
Welcome to Node.js v16.15.1.
Type ".help" for more information.
> s = '('
'('
> s = '\('
'('
> s = '\\('
'\\('
> s = String.raw`(`
'('
> s = String.raw`\(`
'\\('
> s = String.raw`\\(`
'\\\\('
So, how do I set s to '\('?
You don't.
You're confused about the value of a string and its representation (whence Python repr() function).
A parenthesis is not a special character in a JavaScript string, so it will never be escaped in its canonical "code" representation. Therefore you will never make a string whose representation is '\('.
If you mean a string of length 2, with the first character being a backslash, and the second one being a left parenthesis, then '\\(' is what you want.