IntegerToEnglish.js

const Notions = {
    1: 'Thousand',
    2: 'Million',
    3: 'Billion',
}

const UnderTwenty = {
    0: '',
    1: 'One',
    2: 'Two',
    3: 'Three',
    4: 'Four',
    5: 'Five',
    6: 'Six',
    7: 'Seven',
    8: 'Eight',
    9: 'Nine',
    10: 'Ten',
    11: 'Eleven',
    12: 'Twelve',
    13: 'Thirteen',
    14: 'Fourteen',
    15: 'Fifteen',
    16: 'Sixteen',
    17: 'Seventeen',
    18: 'Eighteen',
    19: 'Nineteen',
    20: 'Twenty',
}

const Named = {
  ...UnderTwenty,
  0: 'Zero',
}

const Dozen = {
  2: 'Twenty',
  3: 'Thirty',
  4: 'Forty',
  5: 'Fifty',
  6: 'Sixty',
  7: 'Seventy',
  8: 'Eighty',
  9: 'Ninety',
}

const chunklify = (num) => {
    const arr = num.toString()
    
    const chunks = []

    let count = 1
    let tmp = []

    for (let i = arr.length - 1; i >=0; i--) {
        if (count == 3) {
            tmp.push(arr[i])
            chunks.push(tmp.reverse())

            count = 1
            tmp = []
        } else {
            if (arr[i]) {
                count++;
                tmp.push(arr[i])
            }
            
            if (i == 0) {
                chunks.push(tmp.reverse())
            }
        }
    }
    
    return chunks
}

const trans = (num) => {
  if (num <= 20) {
    return Named[num]
  }
    
  const chunks = chunklify(num);

  const texts = chunks.map((chunk) => {
    let text = ''
    let value = 0
    if (chunk.length == 3) {
      if (UnderTwenty[chunk[0]]) {
        text = `${UnderTwenty[chunk[0]]} Hundred`
      }

      value = parseInt(`${chunk[1]}${chunk[2]}`)

      if (value <= 20) {
        if (text) {
          if (UnderTwenty[value]) {
            text = `${text} ${UnderTwenty[value]}`
          }
        } else {
          text = `${UnderTwenty[value]}`
        }
      } else {
        text = `${text} ${Dozen[chunk[1]]} ${UnderTwenty[chunk[2]]}`
      }
    } else if (chunk.length == 2) {
      value = parseInt(`${chunk[0]}${chunk[1]}`)

      if (value <= 20) {
        text = `${UnderTwenty[value]}`
      } else {
        text = `${Dozen[chunk[0]]} ${UnderTwenty[chunk[1]]}`
      }
    } else {
      value = parseInt(`${chunk[0]}`)
      text = `${UnderTwenty[value]}`
    }

    return text
  })

  const textsWithNotion = texts.map((text, i) => {
    if (i > 0) {
      if (text) {
        return `${text} ${Notions[i]}` 
      }

      return `${text}`
    }

    return `${text}`
  }, '')

  const result = textsWithNotion.reverse().join(' ')

  return result.trim().replace(/\s+/g, ' ')
}

console.log(trans(1000000))