Encode and Decode Strings with Base64 in JavaScript

Google Scripts offer the Utilities.base64Encode() and base64Decode() methods to easily encode and decode strings in base64 respectively. You can do Base64 encoding and decoding in plain Javascript as well. Here’s the un-minified code to help you understand what happens behind the scene.

var Base64 = {
  _keyStr: 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=',
  encode: function (e) {
    var t = '';
    var n, r, i, s, o, u, a;
    var f = 0;
    e = Base64._utf8_encode(e);
    while (f < e.length) {
      n = e.charCodeAt(f++);
      r = e.charCodeAt(f++);
      i = e.charCodeAt(f++);
      s = n >> 2;
      o = ((n & 3) << 4) | (r >> 4);
      u = ((r & 15) << 2) | (i >> 6);
      a = i & 63;
      if (isNaN(r)) {
        u = a = 64;
      } else if (isNaN(i)) {
        a = 64;
      }
      t = t + this._keyStr.charAt(s) + this._keyStr.charAt(o) + this._keyStr.charAt(u) + this._keyStr.charAt(a);
    }
    return t;
  },
  decode: function (e) {
    var t = '';
    var n, r, i;
    var s, o, u, a;
    var f = 0;
    e = e.replace(/[^A-Za-z0-9\+\/\=]/g, '');
    while (f < e.length) {
      s = this._keyStr.indexOf(e.charAt(f++));
      o = this._keyStr.indexOf(e.charAt(f++));
      u = this._keyStr.indexOf(e.charAt(f++));
      a = this._keyStr.indexOf(e.charAt(f++));
      n = (s << 2) | (o >> 4);
      r = ((o & 15) << 4) | (u >> 2);
      i = ((u & 3) << 6) | a;
      t = t + String.fromCharCode(n);
      if (u != 64) {
        t = t + String.fromCharCode(r);
      }
      if (a != 64) {
        t = t + String.fromCharCode(i);
      }
    }
    t = Base64._utf8_decode(t);
    return t;
  },
  _utf8_encode: function (e) {
    e = e.replace(/\r\n/g, '\n');
    var t = '';
    for (var n = 0; n < e.length; n++) {
      var r = e.charCodeAt(n);
      if (r < 128) {
        t += String.fromCharCode(r);
      } else if (r > 127 && r < 2048) {
        t += String.fromCharCode((r >> 6) | 192);
        t += String.fromCharCode((r & 63) | 128);
      } else {
        t += String.fromCharCode((r >> 12) | 224);
        t += String.fromCharCode(((r >> 6) & 63) | 128);
        t += String.fromCharCode((r & 63) | 128);
      }
    }
    return t;
  },
  _utf8_decode: function (e) {
    var t = '';
    var n = 0;
    var r = (c1 = c2 = 0);
    while (n < e.length) {
      r = e.charCodeAt(n);
      if (r < 128) {
        t += String.fromCharCode(r);
        n++;
      } else if (r > 191 && r < 224) {
        c2 = e.charCodeAt(n + 1);
        t += String.fromCharCode(((r & 31) << 6) | (c2 & 63));
        n += 2;
      } else {
        c2 = e.charCodeAt(n + 1);
        c3 = e.charCodeAt(n + 2);
        t += String.fromCharCode(((r & 15) << 12) | ((c2 & 63) << 6) | (c3 & 63));
        n += 3;
      }
    }
    return t;
  },
};

var str = 'The quick brown fox';
console.log(Base64.encode(str));

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