How should the custom mapping function look like for mapping a key onto two different cmp.functions?

[zijian-x]

For example if cmp.visible(), <tab> should map to cmp.mapping.confirm({ select = true}), or if it's not visible then it should invoke the completion menu via cmp.mapping.complete(). The function below somehow doesn't work:

function(_)
  if cmp.visible() then
    cmp.mapping.confirm({ select = true })
  else
    cmp.mapping.complete()
  end
end

I don't have any other existing mapping for <tab>.

[fitrh]

See :h cmp-mapping, at the end of section, it says

Built-in mapping helpers are only available as a configuration option.
If you want to call nvim-cmp features directly, please use |cmp-function| instead.

It means that cmp.mapping is used in the form of key = cmp.mapping.*, using custom callback means you have to write it in the followinhg form

function(_)
  if cmp.visible() then
    cmp.confirm({ select = true })
  else
    cmp.complete()
  end
end

[zijian-x]

Rookie mistake, thanks 👍🏼