base64 in JavaScript

base64_final.js

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

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

Modernizr.load({
    test: window.btoa && window.atob,
    nope: 'base64.js',
    complete: function () {
        var encodedData = utf8_to_b64("Hello, world"); // encode a string
        var decodedData = b64_to_utf8(encodedData); // decode the string
    }
});

base64_unicode.js

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

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

// Usage:
utf8_to_b64('✓ à la mode'); // "4pyTIMOgIGxhIG1vZGU="
b64_to_utf8('4pyTIMOgIGxhIG1vZGU='); // "✓ à la mode"

base64js_using_modernizr.js

Modernizr.load({
    test: window.btoa && window.atob,
    nope: 'base64.js',
    complete: function () {
        // `btoa` and `atob` are now safe to use
    }
});

base64js_using_yepnope.js

yepnope({
    test: window.btoa && window.atob,
    nope: 'base64.js',
    callback: function () {
        // `btoa` and `atob` are now safe to use
    }
});