How to Base64 Encode/Decode in JavaScript, Python, and the Command Line

JavaScript (Browser)

Browsers have built-in btoa() (binary-to-ASCII, encode) and atob() (ASCII-to-binary, decode). They work great for plain ASCII text:

const encoded = btoa('Hello');   // "SGVsbG8="
const decoded = atob('SGVsbG8='); // "Hello"

The catch: btoa() only supports Latin1 characters (code points 0–255) and throws InvalidCharacterError on anything outside that range — which means emoji, Chinese, Japanese, and most non-English text will break it. The fix is to UTF-8 encode the string into bytes first, then Base64-encode those bytes:

function utf8ToB64(str) {
  const bytes = new TextEncoder().encode(str);
  let bin = '';
  bytes.forEach(b => bin += String.fromCharCode(b));
  return btoa(bin);
}
function b64ToUtf8(b64) {
  const bin = atob(b64);
  const bytes = new Uint8Array(bin.length);
  for (let i = 0; i < bin.length; i++) bytes[i] = bin.charCodeAt(i);
  return new TextDecoder().decode(bytes);
}

utf8ToB64('Hello 👋 世界'); // works fine, unlike plain btoa()

Node.js

Node's built-in Buffer class handles Base64 (and Unicode) natively — no workaround needed:

const encoded = Buffer.from('Hello 👋 世界', 'utf-8').toString('base64');
console.log(encoded);

const decoded = Buffer.from(encoded, 'base64').toString('utf-8');
console.log(decoded); // "Hello 👋 世界"

Python

Python's standard library base64 module works on bytes, so you need .encode() before encoding and .decode() after decoding:

import base64

# Encode
text = "Hello 👋 世界"
encoded_bytes = base64.b64encode(text.encode('utf-8'))
encoded_str = encoded_bytes.decode('ascii')
print(encoded_str)

# Decode
decoded_bytes = base64.b64decode(encoded_str)
decoded_str = decoded_bytes.decode('utf-8')
print(decoded_str)  # "Hello 👋 世界"

Command Line

Linux / macOS

Most Unix-like systems ship with a base64 command:

# Encode
echo -n "Hello" | base64
# SGVsbG8=

# Decode
echo -n "SGVsbG8=" | base64 --decode
# Hello (use -D instead of --decode on macOS/BSD)

Windows

Windows doesn't ship a plain base64 command, but certutil (built into every Windows install) can do it, or use PowerShell directly:

:: certutil (writes to/from a file)
certutil -encode input.txt output.b64
certutil -decode output.b64 decoded.txt

# PowerShell (in-memory, no temp file needed)
[Convert]::ToBase64String([Text.Encoding]::UTF8.GetBytes("Hello"))
[Text.Encoding]::UTF8.GetString([Convert]::FromBase64String("SGVsbG8="))

Frequently Asked Questions