Function is_git_builtin 

pub fn is_git_builtin(word: &[u8]) -> bool

What: Whether word is a command built into Git 2.56.0. &[u8] borrows the word’s raw bytes; bool is true or false. Why: A built-in word cannot be an alias, so what the word says is what Git runs.

In TS you’d write (pseudocode):

function isGitBuiltin(word: string): boolean { return GIT_BUILTIN_COMMANDS.has(word); }