Base64 encode & decode (only for numbers) in JavaScript

Base64.js

var Base64 = (function () {

    var ALPHA = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/';

    var Base64 = function () {};

    var _encode = function (value) {

        if (typeof(value) !== 'number') {
            throw 'Value is not number!';
        }

        var result = '', mod;
        do {
            mod = value % 64;
            result = ALPHA.charAt(mod) + result;
            value = Math.floor(value / 64);
        } while(value > 0);

        return result;
    };

    var _decode = function (value) {

        var result = 0;
        for (var i = 0, len = value.length; i < len; i++) {
            result *= 64;
            result += ALPHA.indexOf(value[i]);
        }

        return result;
    };

    Base64.prototype = {
        constructor: Base64,
        encode: _encode,
        decode: _decode
    };

    return Base64;

})();

var base64 = new Base64();
var s = base64.encode(1234567890); // BJlgLS
var n = base64.decode('BJlgLS'); // 1234567890

How do we decode based on the specific value encoded by _encode, A.K.A. setting var n = base64.decode(custom result); as a custom result? (I know this is 5 years later)

the algorithm skip some values :(

at dictionary [a-zA-Z0-9_-] (64 symbols)
your variant has
.encode(63) = -,
.encode(64) = ba, must be aa

The fix is value = ... - 1 and while (value > -1), in encode, and result += Math.pow(64, i) * (ALPHA.indexOf(value[i]) + Math.min(i, 1)) instead of the contents of the for in decode.
.

Using the forementioned fix, this is what I ended up with:

const BASE62 = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789'
const BASE64 = BASE62 + '+/'
const BASE64URL = BASE62 + '-_'

export function numberToBase64(n: number): string {
  return numberToBase(n, BASE64)
}

export function numberToBase64url(n: number): string {
  return numberToBase(n, BASE64URL)
}

function numberToBase(n: number, alphabet: string): string {
  const alen = alphabet.length
  let result = ''

  do {
    result = alphabet.charAt(n % alen) + result
    n = Math.floor(n / alen) - 1
  } while(n > -1)

  return result
}