Base64 is a group of similar binary-to-text encoding schemes that represent binary data in an ASCII string format by translating it into a radix-64 representation. The term Base64 originates from a specific MIME content transfer encoding.

Base64 encoding schemes are commonly used when there is a need to encode binary data that needs to be stored and transferred over media that are designed to deal with textual data. This is to ensure that the data remain intact without modification during transport. Base64 is commonly used in a number of applications including email via MIME, and storing complex data in XML.

In JavaScript there are two functions respectively for decoding and encoding base64 strings:

The atob() function decodes a string of data which has been encoded using base-64 encoding. Conversely, the btoa() function creates a base-64 encoded ASCII string from a "string" of binary data.

Both atob() and btoa() work on strings. If you want to work on ArrayBuffers, please, read this paragraph.


data URIs
data URIs, defined by RFC 2397, allow content creators to embed small files inline in documents.
Wikipedia article about Base64 encoding.
Decodes a string of data which has been encoded using base-64 encoding.
Creates a base-64 encoded ASCII string from a "string" of binary data.
The "Unicode Problem"
In most browsers, calling btoa() on a Unicode string will cause a Character Out Of Range exception. This paragraph shows some solutions.
List of Mozilla supported URI schemes
In this article is published a library of ours whose aims are:
  • creating a C-like interface for strings (i.e. array of characters codes — ArrayBufferView in JavaScript) based upon the JavaScript ArrayBuffer interface,
  • creating a collection of methods for such string-like objects (since now: stringViews) which work strictly on array of numbers rather than on immutable JavaScript strings,
  • working with other Unicode encodings, different from default JavaScript's UTF-16 DOMStrings,

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The "Unicode Problem"

Since DOMStrings are 16-bit-encoded strings, in most browsers calling window.btoa on a Unicode string will cause a Character Out Of Range exception if a character exceeds the range of a 8-bit byte (0x00~0xFF). There are two possible methods to solve this problem:

  • the first one is to escape the whole string (with UTF-8, see encodeURIComponent) and then encode it;
  • the second one is to convert the UTF-16 DOMString to an UTF-8 array of characters and then encode it.

Here are the two possible methods.

Solution #1 – escaping the string before encoding it

function b64EncodeUnicode(str) {
    // first we use encodeURIComponent to get percent-encoded UTF-8,
    // then we convert the percent encodings into raw bytes which
    // can be fed into btoa.
    return btoa(encodeURIComponent(str).replace(/%([0-9A-F]{2})/g,
        function toSolidBytes(match, p1) {
            return String.fromCharCode('0x' + p1);

b64EncodeUnicode('✓ à la mode'); // "4pyTIMOgIGxhIG1vZGU="
b64EncodeUnicode('\n'); // "Cg=="

To decode the Base64-encoded value back into a String:

function b64DecodeUnicode(str) {
    // Going backwards: from bytestream, to percent-encoding, to original string.
    return decodeURIComponent(atob(str).split('').map(function(c) {
        return '%' + ('00' + c.charCodeAt(0).toString(16)).slice(-2);

b64DecodeUnicode('4pyTIMOgIGxhIG1vZGU='); // "✓ à la mode"
b64DecodeUnicode('Cg=='); // "\n"

Unibabel implements common conversions using this strategy.

Solution #2 – rewrite the DOMs atob() and btoa() using JavaScript's TypedArrays and UTF-8

Use a TextEncoder polyfill such as TextEncoding (also includes legacy windows, mac, and ISO encodings), TextEncoderLite, combined with a Buffer and a Base64 implementation such as base64-js.

When a native TextEncoder implementation is not available, the most light-weight solution would be to use TextEncoderLite with base64-js. Use the browser implementation when you can.

The following function implements such a strategy. It assumes base64-js imported as <script type="text/javascript" src="base64js.min.js"/>. Note that TextEncoderLite only works with UTF-8.

function Base64Encode(str, encoding = 'utf-8') {
    var bytes = new (TextEncoder || TextEncoderLite)(encoding).encode(str);        
    return base64js.fromByteArray(bytes);

function Base64Decode(str, encoding = 'utf-8') {
    var bytes = base64js.toByteArray(str);
    return new (TextDecoder || TextDecoderLite)(encoding).decode(bytes);

In some cases, the above conversion to UTF-8 and then to Base64 will not be very space efficient. UTF-8 produces longer output than UTF-16 when the text contains a large percentage of characters in the range U+0800-U+FFFF, which are encoded with three bytes in UTF-8 but two in UTF-16. In the case where the JavaScript string contains evenly-distributed UTF-16 code points, one might consider encoding to UTF-16 instead of UTF-8 before the conversion to Base64, for a 40% reduction in size.