How to encode Base64 in JavaScript

In the browser, use btoa() to encode: const encoded = btoa('Hello World'). For Node.js, use Buffer: const encoded = Buffer.from('Hello World').toString('base64'). Important: btoa() only handles Latin-1 characters. For Unicode strings, first encode to UTF-8: const encoded = btoa(unescape(encodeURIComponent(unicodeString))). Or in modern environments: const encoded = btoa(new TextEncoder().encode(str).reduce((s, b) => s + String.fromCharCode(b), '')).

// JavaScript — Base64 encode and decode

// Encode string to Base64
const encoded = btoa("Hello, World!");
console.log(encoded); // "SGVsbG8sIFdvcmxkIQ=="

// Decode Base64 to string
const decoded = atob("SGVsbG8sIFdvcmxkIQ==");
console.log(decoded); // "Hello, World!"

// Handle Unicode characters
const unicodeEncode = btoa(unescape(encodeURIComponent("Héllo 🌍")));
const unicodeDecode = decodeURIComponent(escape(atob(unicodeEncode)));

// Node.js
Buffer.from("Hello").toString("base64");     // encode
Buffer.from("SGVsbG8=", "base64").toString(); // decode

How to decode Base64 in JavaScript

Browser decoding: const decoded = atob(encodedString). Node.js: const decoded = Buffer.from(encodedString, 'base64').toString('utf-8'). For Base64URL (used in JWTs and URLs), replace - with + and _ with / before decoding, or use a library. Common error: 'InvalidCharacterError: The string to be decoded is not correctly encoded' — this means your Base64 string has invalid characters or incorrect padding. Use this tool to verify your Base64 string is valid before decoding in code.

Base64 in JavaScript: browser vs Node.js

Browser JavaScript uses btoa()/atob() (Binary to ASCII and ASCII to Binary). Node.js uses Buffer.from(str, 'base64') and buffer.toString('base64'). In Deno and Bun, both APIs are available. For isomorphic code that works everywhere, consider the base64-js npm package or a polyfill. The Fetch API also supports Base64 via: await (await fetch('data:;base64,' + encoded)).text(). For file uploads, FileReader.readAsDataURL() returns a Base64 data URL automatically.