I've tried defining a command with a single required argument, and attempted to validate that exactly one source specified:

import * as program from 'commander';
import { version } from 'pjson';

program
  .version(version)
  .arguments('<source.md>')
  .action((source: string): void => {
    console.log('source=' + source);
    program.source = source;
  })
  .parse(process.argv);

console.log(program.args);
console.log(program.args.length);

if (program.args.length !== 1) {
  console.log('Invalid arguments');
  program.help();
}

console.log(program.source);

And running the program with no arguments, correctly prints 'Invalid arguments' since program.args.length is 0. But with 1 argument (program a) program.args.length is 2!? and outputs the following, claiming that program.args.length is 2!?

source=a
[
  'a',
  Command {
    ...
  }
]
2
Invalid arguments

Weirdly running with four arguments program a b c d also appears to drop the b argument somehow:

source=a
[
  'a',
  Command {
    ...
  },
  'c',
  'd'
]
4
Invalid arguments

So I guess I have two questions:

1. Where's the second argument gone?
2. (more importantly) how can I ensure that exactly one argument has been supplied?