This question has some helpful info: How to do Base64 encoding in node.js?
The Buffer class itself does the conversion:
var base64data = Buffer.from('some binary data', 'binary').toString('base64');
console.log(base64data);
// -> 'c29tZSBiaW5hcnkgZGF0YQ=='
var originaldata = Buffer.from(base64data, 'base64');
console.log(originaldata);
// -> <Buffer 73 6f 6d 65 20 62 69 6e 61 72 79 20 64 61 74 61>
console.log(originaldata.toString());
// -> some binary data