Function command_tokens 

pub fn command_tokens(
    arguments: &[OsString],
) -> Option<(&OsString, &[OsString])>

What: The command word and the tokens after it, when the arguments name a command. Option<(&OsString, &[OsString])> is “a pair of borrowed views, or nothing”: the word, and the list of everything after it. Why: Every rule that reads a command’s own options needs exactly this split. Taking it from one function means no rule computes an offset of its own, so no rule can read the command word as one of its options by an arithmetic slip.

In TS you’d write (pseudocode):

function commandTokens(args: string[]): [word: string, region: string[]] | undefined;