Кодирование и декодирование в формате Base64

data URIs

data URIs, описанные в RFC 2397, позволяют создателям контента встроить в документ маленькие файлы в виде строки (инлайном).

Base64

Wikipedia article about Base64 encoding.

atob()

Decodes a string of data which has been encoded using base-64 encoding.

btoa()

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.

URIScheme

List of Mozilla supported URI schemes

StringView

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,

• Rewriting atob() and btoa() using TypedArrays and UTF-8
• StringView – a C-like representation of strings based on typed arrays

• ArrayBuffer
• Typed arrays
• ArrayBufferView
• Uint8Array
• StringView – a C-like representation of strings based on typed arrays
• DOMString
• URI
• encodeURI()

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.

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);
      })
      .join(""),
  );
}

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

Unibabel implements common conversions using this strategy.

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);
}