What does `_` do in this map() expression?

Question

I'm trying to understand the code suggested in https://vi.stackexchange.com/a/28506/6189:

function MyLinks(pat, spat, ssub, sflags) abort
  return glob(a:pat, v:false, v:true)
        \ ->map({_, v -> printf('[%s](%s)', fnamemodify(v, ':t:r')->substitute(a:spat, a:ssub, a:sflags), fnamemodify(v, ':t'))})
        \ ->join("\n")
endfunction

...and this map({_,... syntax confuses me.

:h map() says: map({expr1}, {expr2}) and states: "Replace each item in {expr1} with the result of evaluating {expr2}".

So map is using the input from glob(...) as its first argument, and this {long..bunch..of..stuff} is the second argument. That makes sense. But "long bunch of stuff" begins with {_,. What is that? What is _?

Answer 1

See my answer to your other question about v: What is `v` in this vimscript?

_ is the chosen name for the key/index parameter of the lambda. As it isn't used, _ is used to say ignored/unused.