random number using node crypto

random.js

'use strict'

const assert = require('assert')
const crypto = require('crypto')

module.exports = (min, max) => {

  // borrowed from http://stackoverflow.com/a/33627342

  // figure out range and how many bytes required
  const range = max - min
  const numBits = Math.log(range) / Math.log(2)
  const numBytes = Math.floor(numBits / 8) + 1
  const maxRange = Math.pow(256, numBytes)

  // if min === max, don't proceed, just return the value
  // numBits is -Infinity and crypto doesn't like it.
  if (range === 0) {
    return min
  }

  // perform some sanity checks on the inputs
  assert(typeof min === 'number' && !isNaN(min), 'min must be a number')
  assert(typeof max === 'number' && !isNaN(max), 'max must be a number')
  assert(min <= max, 'min must be less than or equal to max')
  assert(Number.isSafeInteger(max), 'max must be a safe integer')
  assert(numBytes < 6, 'cant calculate when range >= 6 bytes')

  // calculate the random bytes, turn into integer
  const bytes = crypto.randomBytes(numBytes)
  const num = parseInt(bytes.toString('hex'), 16)
  return Math.floor(num / maxRange * (range + 1) + min)
}