How to Decode Base64 Strings in JavaScript While Handling UTF-8 Encoding?

Decode Base64 Using JavaScript atob Function: Handling UTF-8

JavaScript's atob() function is designed to decode base64-encoded strings. Users may encounter issues when decoding UTF-8 encoded strings, resulting in ASCII-encoded characters instead of proper UTF-8 representation.

Challenge: Understanding the Unicode Problem

Base64 expects binary data as input, and JavaScript considers strings with characters occupying one byte as binary data. Characters occupying more than one byte in UTF-8 encoded strings, however, trigger exceptions during encoding.

Solution 1: Binary Interoperability

The recommended fix is to encode to and decode binary strings:

Encoding UTF-8 to Binary

function toBinary(string) {
  const codeUnits = new Uint16Array(string.length);
  for (let i = 0; i <p><strong>Decoding Binary to UTF-8</strong></p><pre class="brush:php;toolbar:false">function fromBinary(encoded) {
  const binary = atob(encoded);
  const bytes = new Uint8Array(binary.length);
  for (let i = 0; i <p>This solution converts the original UTF-8 string to a binary representation, preserving UTF-16 encoding, a native representation in JavaScript.</p><h3 id="Solution-ASCII-Base-Interoperability">Solution 2: ASCII Base64 Interoperability</h3><p>An alternative solution focused on UTF-8 interoperability is to maintain plaintext base64 strings:</p><p><strong>Encoding UTF-8 to Base64</strong></p><pre class="brush:php;toolbar:false">function b64EncodeUnicode(str) {    
  return btoa(encodeURIComponent(str).replace(/%([0-9A-F]{2})/g,
    function toSolidBytes(match, p1) {
      return String.fromCharCode('0x' + p1);
  }));
}

Decoding Base64 to UTF-8

function b64DecodeUnicode(str) {
  return decodeURIComponent(atob(str).split('').map(function(c) {
    return '%' + ('00' + c.charCodeAt(0).toString(16)).slice(-2);
  }).join(''));
}

This solution efficiently handles UTF-8 encoded strings without altering their representation.

TypeScript Support

// Encoding UTF-8 ⇢ base64

function b64EncodeUnicode(str) {
    return btoa(encodeURIComponent(str).replace(/%([0-9A-F]{2})/g, function(match, p1) {
        return String.fromCharCode(parseInt(p1, 16))
    }))
}

// Decoding base64 ⇢ UTF-8

function b64DecodeUnicode(str) {
    return decodeURIComponent(Array.prototype.map.call(atob(str), function(c) {
        return '%' + ('00' + c.charCodeAt(0).toString(16)).slice(-2)
    }).join(''))
}

Historical Solution (Deprecated)

function utf8_to_b64( str ) {
  return window.btoa(unescape(encodeURIComponent( str )));
}

function b64_to_utf8( str ) {
  return decodeURIComponent(escape(window.atob( str )));
}

While still functional, this approach is now deprecated in modern browsers.

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