Tiny ARGV Parser in Node

tiny-argv.js

/**
  Usage:
 
  const arguments = argv()
    .value({key: 'foo', defaultValue: 'bar', alias: 'f'})
    .flag({key: 'flag'})
  console.log(arguments.foo, arguments.flag)  
  
  > node script.js --flag -f TinyArgv
  TinyArgv true
  
  > node script.js -f "Tiny Argv"
  Tiny Argv false

  > node script.js --foo="Tiny Argv"
  Tiny Argv false

  Other arguments are collected on _:

  const arguments = argv()
    .flag({key: 'flag'})
  console.log(arguments.flag, arguments._)

  > node script.js --flag file1 file2
  true [ 'file1', 'file2' ]

*/
class TinyArgv {
  constructor(argv) {
    this.__argv = argv
    this.__cleared = []
    this._ = []
  }
  /**
   * Rebuild remainder based on indices that have been read.
   */
  __rebuildUnderscore() {
    if (this.__cleared.length === 0) return
    this._ = this.__argv.filter((_arg, i) => this.__cleared.indexOf(i) < 0)
  }
  /**
   * Mark an index as processed.
   */
  __clear(index) {
    if (index >= 0 && index < this.__argv.length) {
      this.__cleared.push(index)
      this.__rebuildUnderscore()
    }
  }
  /**
   * Figure out the location of an argument in ARGV based on a key and an alias.
   * Returns -1 if neither is found.
   */
  __argIndex(key, alias) {
    const keyTest = new RegExp(`^--${key}`)
    const aliasTest = alias ? new RegExp(`^-${alias}`) : undefined
    return this.__argv.reduce((acc, cur, index) => {
      if (acc >= 0) return acc
      if (keyTest.test(cur)) return index
      if (alias && aliasTest.test(cur)) return index
    }, -1)
  }
  /**
   * Extracts the value of a switch, which may be more than one word.
   */
  __extractValue(index) {
    if (this.__argv[index].indexOf('=') >= 0) {
      const [, value] = this.__argv[index].split('=')
      return value;
    }
    this.__clear(index + 1)
    return this.__argv[index + 1]
  }
  /**
   * Extract a flag (boolean) argument.
   * Can have a single-character alias and a default value (so can be true instead
   * of false).
   */
  flag({key, defaultValue = false, alias = false}) {
    const indexOf = this.__argIndex(key, alias)
    this.__clear(indexOf)
    this[key] = indexOf >= 0 ? !defaultValue : defaultValue
    return this;
  }
  /*
   * Extract a string-based argument.
   * Can have a single-character alias or a default value.
   */
  value({key, defaultValue = null, alias = false}) {
    const indexOf = this.__argIndex(key, alias)
    this.__clear(indexOf)
    this[key] = indexOf >= 0 ? this.__extractValue(indexOf) : defaultValue
    return this;
  }
}
/**
 * Convenient chain-initiating function. Call this to start the processing.
 * Optional argument: can process a different array than process.argv
 */
const argv = function (argv) {
  return new TinyArgv(argv ? argv : process.argv.slice(2))
}

module.exports = argv