Flags with optional values

[JuanM04]

Is there a way to have optional flag values? Something like the following:

const arg = require('arg');

const args = arg({
	'--port': Number, // --port <number>
	'--host': String.optional, // --host [string]
});

Given --port 5432 --host localhost it should return { _: [], port: 5432, host: 'localhost' },
given --port 5432 --host it should return something like { _: [], port: 5432, host: true },
and given --port 5432 it should return { _: [], port: 5432 },

[cspotcode]

I'd like this for ts-node to emulate the behavior of node's -p flag. node -p 123 is equivalent to node -pe 123

[psychobolt]

Workaround is to pass the permissive flag and use a string type, then find the arg value manually:

// cmd.js
const defaultSince = 'origin/main';

const { _ = [], ..args } = arg({
  '--since': defaultSince
}, { permissive: true });

console.log(_);

const sinceIndex = _.indexOf('--since');
if (sinceIndex > -1) {
  console.log(_[sinceIndex + 1]);
}

node cmd.js --since develop

# [ '--since', 'develop' ]
# develop

[bybraveHQ]

Added in the maintained fork @bybrave/arg2: arg.optional(type).

const spec = { '--host': arg.optional(String) };
arg(spec, { argv: ['--host', 'localhost'] }); // { _: [], '--host': 'localhost' }
arg(spec, { argv: ['--host'] });              // { _: [], '--host': true }
arg(spec, { argv: [] });                      // { _: [] }

With a value it parses normally; without one (end of argv or followed by another option) it resolves to true, like a flag. npm i @bybrave/arg2.