Underscore as argument to javascript function?

Question

I always see this:

(_, arg1, arg2) => some expresion

What does the underscore entail? I assume a 'we don't really mind'.

Answer 1

This is a common idiom used to indicate an unused argument.

The convention differs slightly between communities: Erlang commonly suffixes the variable used in the function signature, Prolog adds a single differentiating character while OCaml omits all but the character itself. All are widely understood pieces of metonymy in computer programming.