fast-luhn/index.js

A fast Luhn algorithm for validating credit cards

'use strict'

const lookup = [0, 2, 4, 6, 8, 1, 3, 5, 7, 9]

module.exports = function luhn (number) {
  if (typeof number !== 'string') throw new TypeError('Expected string input')
  if (!number) return false

  let index = number.length
  let x2 = true
  let sum = 0

  while (index) {
    const value = number.charCodeAt(--index) - 48
    if (value < 0 || value > 9) return false

    x2 = !x2
    sum += x2 ? lookup[value] : value
  }

  return sum % 10 === 0
}