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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
    }
});