As of Node.js v6.0.0 using the constructor method has been deprecated and the following method should instead be used to construct a new buffer from a base64 encoded string:

var b64string = /* whatever */;
var buf = Buffer.from(b64string, 'base64'); // Ta-da

For Node.js v5.11.1 and below

Construct a new Buffer and pass 'base64' as the second argument:

var b64string = /* whatever */;
var buf = new Buffer(b64string, 'base64'); // Ta-da

If you want to be clean, you can check whether from exists :

if (typeof Buffer.from === "function") {
    // Node 5.10+
    buf = Buffer.from(b64string, 'base64'); // Ta-da
} else {
    // older Node versions, now deprecated
    buf = new Buffer(b64string, 'base64'); // Ta-da
}

Buffers can be used for taking a string or piece of data and doing Base64 encoding of the result. For example:

> console.log(Buffer.from("Hello World").toString('base64'));
SGVsbG8gV29ybGQ=
> console.log(Buffer.from("SGVsbG8gV29ybGQ=", 'base64').toString('ascii'))
Hello World

The Buffer constructor is a global object, so no require is needed. Buffers created with strings can take an optional encoding parameter to specify what encoding the string is in. The available toString and Buffer constructor encodings are as follows:

'ascii' - for 7 bit ASCII data only. This encoding method is very fast, and will strip the high bit if set.

'utf8' - Multi byte encoded Unicode characters. Many web pages and other document formats use UTF-8.

'ucs2' - 2-bytes, little endian encoded Unicode characters. It can encode only BMP(Basic Multilingual Plane, U+0000 - U+FFFF).

'base64' - Base64 string encoding.

'binary' - A way of encoding raw binary data into strings by using only the first 8 bits of each character. This encoding method is deprecated and should be avoided in favor of Buffer objects where possible. This encoding will be removed in future versions of Node.

function base64ToArrayBuffer(base64) {
    var binaryString = atob(base64);
    var bytes = new Uint8Array(binaryString.length);
    for (var i = 0; i < binaryString.length; i++) {
        bytes[i] = binaryString.charCodeAt(i);
    }
    return bytes.buffer;
}

new Buffer(...) has been deprecated for a while now, go for Buffer.from(...)

a simple example might be:

var utf8encoded = Buffer.from(base64encoded, 'base64').toString('utf8');