Creates a base-64 encoded ASCII string from a "string" of binary data.

Please note that this is not suitable for raw Unicode strings! See Unicode section below.


var encodedData = window.btoa(stringToEncode);


var encodedData = window.btoa("Hello, world"); // encode a string
var decodedData = window.atob(encodedData); // decode the string


You can use this method to encode data which may otherwise cause communication problems, transmit it, then use the window.atob method to decode the data again. For example, you can encode control characters such as ASCII values 0 through 31.

btoa() is also available to XPCOM components implemented in JavaScript, even though window is not the global object in components.

Unicode Strings

In most browsers, calling window.btoa on a Unicode string will cause a Character Out Of Range exception.

To avoid this, consider this pattern, noted by Johan Sundström:

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

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

// Usage:
utf8_to_b64('✓ à la mode'); // JTI1dTI3MTMlMjUyMCUyNUUwJTI1MjBsYSUyNTIwbW9kZQ==
b64_to_utf8('JTI1dTI3MTMlMjUyMCUyNUUwJTI1MjBsYSUyNTIwbW9kZQ=='); // "✓ à la mode"

utf8_to_b64('I \u2661 Unicode!'); // SSUyNTIwJTI1dTI2NjElMjUyMFVuaWNvZGUlMjUyMQ==
b64_to_utf8('SSUyNTIwJTI1dTI2NjElMjUyMFVuaWNvZGUlMjUyMQ=='); // "I ♡ Unicode!"

A better, more faithful and less expensive solution is to convert the DOMString to a UTF-8 encoded string passing for typed arrays. In order to do this, please, read this paragraph.


DOM Level 0. Not part of any standard. Except of course

Browser compatibility

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari (WebKit)
Basic support (Yes) (Yes) 10 (Yes) (Yes)
Feature Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support (Yes) ? Not supported ? (Yes)

See also