Dynamic completions

[daguej]

It would be nice if we could generate dynamic completions/choices on-the-fly when the user presses Tab in their shell. I would like to complete a positional argument or option value that's not from a hardcoded list, but eg loaded from a database.

For example, something like:

yargs(…).command('foo <title>', '…', (yargs) =>
  yargs.positional('title', {
    choices: async (str) => {
      const results = await database.findMany({ where: { title: { startsWith: str }}})
      return results //, an array of strings
    },
  }),
  async (argv) => { … }
);

Typing app foo ba Tab would invoke choices('ba'), which could return ['bar'], and ultimately the shell would complete the title argument with a value found in a database.

[shadowspawn]

Possibly related: #831 (comment) #2298