Encode and Decode Strings with Base64 in JavaScript

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Published 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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Published in: Javascript

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Web Geek, Google Developer Expert
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Amit Agarwal

Amit Agarwal is a Google Developer Expert in Google Workspace and Google Apps Script. He holds an engineering degree in Computer Science (I.I.T.) and is the first professional blogger in India. He is the developer of Mail Merge for Gmail and Document Studio. Read more on Lifehacker and YourStory

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