argument-parser/src/index.ts

import initState from './initState';
import IOption from './IOption';
import parseArgument from './parseArgument';
import runPreParseHook from './runPreParseHook';

/**
 * Checks the command line arguments against the given specification, and returns the arguments as a JavaScript object, as well as a failure function that prints the usage instructions before terminating.
 * If an error is found, outputs an error message and terminates with a non-zero exit code.
 * @param spec A specification of what arguments are expected.
 * @param preParseHook Optionally, a function to modify the command line arguments before parsing them.
 */
export default function parseArguments(
    spec: { [key: string]: IOption },
    preParseHook?: (args: string[], fail: (message: string) => void) => (string[] | undefined),
  ): { fail: (errorMessage: string) => never, args: { [key: string]: string } } {
  //Initialize state
  const { outputArgs, shortToLongLookup, requiredOptions, fail } = initState(spec);
  const args = runPreParseHook({ args: process.argv.slice(2), preParseHook, fail });

  //Iterate through all command line arguments. When we have read both name and value, verify the argument for correctness.
  //Show error if a name has no value afterwards, or a value has no name in front of it.
  const lastOption = args.reduce(parseArgument.bind(null, { spec, fail, outputArgs, requiredOptions, shortToLongLookup }));

  // argumentName must be cleared to '' after the value is read, so if it is not an empty string, the value was missing
  if (lastOption !== '') {
    return fail(`Option "${lastOption}" was not followed by a value.`);
  }

  //check if any entry in requiredArguments was not set
  for (const optName in requiredOptions) {
    if (spec.hasOwnProperty(optName) && requiredOptions[optName] === false) {
      return fail(`Missing option "${optName}" even though it is required.`);
    }
  }

  return {
    fail,
    args: outputArgs,
  };
}