Encrypt number to string with provided radix chars

Encrypt.js

class Encrypt {
  static create(chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789') {
    const radix = chars.length;
    function encode(num) {
      if (num < radix) {
        return chars.charAt(num);
      }
      return `${encode(Math.floor(num / radix))}${chars.charAt(num % radix)}`;
    }
    function decode(str) {
      const len = str.length;
      let num = 0;
      for (let i = 0; i < len; i++) {
        num += chars.indexOf(str.charAt(len - i - 1)) * Math.pow(radix, i);
      }
      return num;
    }
    return {
      encode,
      decode,
    };
  }
}

encode(Date.now() + Math.floor(Math.random() * 1e3) * 1e13)