Base64 デコード

ブラウザー標準 API で英数字中心の Base64 をデコードできます。UTF-8 には `TextDecoder` が必要です。

// Standard Base64 decode (plain English / Latin-1)
function b64Decode(str) {
  try {
    return atob(str);
  } catch (e) {
    return 'Decode failed: ' + e.message;
  }
}

console.log(b64Decode('SGVsbG8gV29ybGQ=')); // "Hello World"

// UTF-8 decode (recommended)
function b64DecodeUtf8(str) {
  const binary = atob(str);
  const bytes = new Uint8Array(binary.length);
  for (let i = 0; i < binary.length; i++) {
    bytes[i] = binary.charCodeAt(i);
  }
  return new TextDecoder('utf-8').decode(bytes);
}

console.log(b64DecodeUtf8('5LiW55WM')); // UTF-8 text

Python 標準の `base64` ライブラリは Standard と URL-Safe の両方に対応します。

import base64

# Standard decode
encoded = 'SGVsbG8g5LiW55WM'
decoded = base64.b64decode(encoded).decode('utf-8')
print(decoded)

# URL-Safe decode
url_encoded = 'SGVsbG8g5LiW55WM'
decoded = base64.urlsafe_b64decode(url_encoded + '=' * (4 - len(url_encoded) % 4))
print(decoded.decode('utf-8'))

# MIME decode (after removing line breaks)
mime_encoded = 'SGVs\nbG8g\nV29y\nbGQ='
decoded = base64.b64decode(mime_encoded.replace('\n', '')).decode('utf-8')
print(decoded)

# Ignore invalid characters automatically
import re
cleaned = re.sub(r'[^A-Za-z0-9+/=]', '', encoded)
decoded = base64.b64decode(cleaned).decode('utf-8')
print(decoded)

Java 標準ライブラリは RFC 4648、URL-Safe、MIME の Base64 を一通りサポートします。

import java.nio.charset.StandardCharsets;
import java.util.Base64;

public class Base64DecodeExample {
    public static void main(String[] args) {
        String encoded = "SGVsbG8g5LiW55WM";

        // Standard decode
        byte[] decoded = Base64.getDecoder().decode(encoded);
        System.out.println(new String(decoded, StandardCharsets.UTF_8));

        // URL-Safe decode
        byte[] urlDecoded = Base64.getUrlDecoder().decode(encoded);
        System.out.println(new String(urlDecoded, StandardCharsets.UTF_8));

        // MIME decode (handles line breaks automatically)
        String mime = "SGVs\nbG8g\nV29y\nbGQ=";
        byte[] mimeDecoded = Base64.getMimeDecoder().decode(mime);
        System.out.println(new String(mimeDecoded, StandardCharsets.UTF_8));

        // Error handling
        try {
            Base64.getDecoder().decode("invalid@base64");
        } catch (IllegalArgumentException e) {
            System.err.println("Decode failed: " + e.getMessage());
        }
    }
}