Read a command line option

read-option.js

/**
 * Read a command line option. Option names must start with '-'.
 * 
 * Options:
 * 
 * `required`     Determines if the option must appear on the command line.
 * 
 * `multi`        Determines if the option can be specified more than once on the
 *                command line. Causes the return value to be an array.
 * 
 * `converter`    The conversion function to use to convert an option's value.
 *
 * @example
 * const args = process.args.slice(2)
 * const flavors = readOption([ '--flavor' ], args, { multi: true })
 * @template T
 * @param {string[]} names The valid option names on the command line
 * @param {string[]} args The command line arguments without the program and script name
 * @param {{ required?: boolean, multi?: boolean, converter: (value:string) => T }} [options]
 * @return {T|T[]}
 */
const readOption = (names, args, { required = false, multi = false, converter = x => x } = {}) => {
  const values = []

  names.forEach(name => {
    const index = args.findIndex(arg => arg.split('=')[0] === name)

    if (index >= 0) {
      let value = args[index].indexOf('=') > 0
        ? args[index].split('=').slice(1).join('=')
        : (args[index + 1] || '')

      if (value.startsWith('-')) {
        value = ''
      }

      values.push(value)
    }
  })

  if (required && !values.length) {
    throw new Error(`Command line option ${names[0]} is required`)
  }

  if (!multi && values.length > 1) {
    throw new Error(`Command line option ${names[0]} does not support mulitple values`)
  }

  const convertedValues = values.map(converter)
  const value = multi ? convertedValues : convertedValues.pop()

  return value
}
/**
 * Read a command line flag. Flag names must start with '-'.
 * 
 * @example
 * const args = process.args.slice(2)
 * const reportOnly = readOption.flag([ '--reportyOnly' ], args)
 * @param {string[]} names The valid option names on the command line
 * @param {string[]} args The command line arguments without the program and script name
 * @param {{ required?: boolean }} [options]
 * @return {boolean}
 */
readOption.flag = (names, args, { required = false } = {}) => {
  const converter = value => {
    if (value) {
      throw new Error(`Command line option ${names[0]} is a flag and cannot have a value`)
    }
    return true
  }
  const options = { required, multi: false, converter }
  return readOption(names, args, options) || false
}