Roman Number array creation

helpers.js

module.exports = {
  createArray: (begin, end) => {
    const result = []
    for (let i = begin; i<end+1; i++){
      result.push(i)
    }
    return result
  },
  fromRoman: (_str) => {
    var str = _str.toString()
    var result = 0
    var decimal = [1000, 900, 500, 400, 100, 90, 50, 40, 10, 9, 5, 4, 1]
    var roman = ["M", "CM","D","CD","C", "XC", "L", "XL", "X","IX","V","IV","I"]
    for (var i = 0; i <= decimal.length; i++) {
      while (str.indexOf(roman[i]) === 0) {
        result += decimal[i]
        str = str.replace(roman[i],'')
      }
    }
    return result;
  }
}

index.js

/**
 * Roman numbers array
 *
 * 0..IV -> [0, 1, 2, 3, 4]
 * 0..V  -> [0, 1, 2, 3, 4, 5]
 * 0..X  -> [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
 */ 

const assert = require('assert')
const {fromRoman, createArray} = require('./helpers')

// tests for `fromRoman` help function
assert.equal(fromRoman('II'), 2)
assert.equal(fromRoman('IV'), 4)

// test for `createArray` help function
assert.deepEqual(createArray(4, 6), [4, 5, 6])

const proxyHandler = {
  get: function(target, name, receiver) {
    const romanValue = fromRoman(name);
    if (romanValue) {
      return createArray(receiver.valueOf(), romanValue)
    } else {
      return target[name]
    }
  }
}

Number.prototype.__proto__ = new Proxy(Number.prototype.__proto__, proxyHandler)

assert.deepEqual(0..IV, [0, 1, 2, 3, 4,])
assert.deepEqual(0..V,  [0, 1, 2, 3, 4, 5])
assert.deepEqual(0..X,  [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10])