Factorial with String

factorial.js

const factorial = number => {
  let f = 1
  for (let i = 1; i <= number; i++) {
    console.time('elapsed time')
    f *= i;
    console.log(`${i}! = ${f}`)
    console.timeEnd('elapsed time')
    if (f === Infinity)
      break;
  }
  return f
}

const factorialString = number => {
  let numberAsArray = asArray(number)
  let f = asArray(1)
  for (let i = 1; i <= number; i++) {
    console.time('elapsed time')
    f = multiply2StringArrays(f, asArray(i))
    console.log(`${i}! = ${arrayToString(f)}`)
    console.timeEnd('elapsed time')
  }
  return arrayToString(f)
}

const asArray = n => [...String(n)]

const multiply2StringArrays = (leftArray, rightArray) => {
  let result = []
  for (let i = 0; i < leftArray.length; i++) {
    for (let j = 0; j < rightArray.length; j++) {
      const leftChar = leftArray[i]
      const rightChar = rightArray[j]
      const sum = String(Number(leftChar) * Number(rightChar))
      const position = (leftArray.length - i) + (rightArray.length - j) + sum.length - 2
      const value = asArray(sum.padEnd(position, '0'))
      result = [...result, value]
    }
  }
  return sumStringArrays(result)
}

const sumStringArrays = arrays => arrays.reduce((acc, value) => sum2StringArrays(acc, value), [])

const sum2StringArrays = (leftArray, rightArray) => {
  let result = []
  let maxSize = Math.max(leftArray.length, rightArray.length)

  leftArray = leftPadArray(leftArray, maxSize)
  rightArray =  leftPadArray(rightArray, maxSize)

  for (let i = maxSize - 1; i >= 0; i--) {
    let leftValue = 0
    let rightValue = 0

    if (i < leftArray.length) {
      leftValue = leftArray[i]
    }
    if (i < rightArray.length) {
      rightValue = rightArray[i]
    }
    
    const value = String(Number(leftValue) + Number(rightValue))
    result = [value, ...result]
  }
  return normalizeArray(result)
}

const shouldNormalize = array => array.filter(item => [...item].length > 1).length > 0

const leftPadArray = (array, size) => [...('0'.repeat(size - array.length)), ...array]

const rightPadArray = (array, size) => [...array, ...('0'.repeat(size - array.length))]

const arrayToString = array => array.join('')

const normalizeArray = (array) => {
  if (!shouldNormalize(array))
    return array

  let result = []
  for (let i = array.length - 1; i >= 0; i--) {
    const value = array[i]
    const pad = [...value].length + array.length - i - 1
    const valueAsArray = rightPadArray([...value], pad)
    result = sum2StringArrays(result, valueAsArray)
  }
  return result
}

console.log(
  factorial(99999)
)

console.log(
  factorialString(99999)
)