Decode Facebook signed_request with NodeJS 8+

fb_decode.js

function parse_signed_request(signed_request, secret) {
    const encoded_data = signed_request.split(".",2);
    // decode the data
    const sig = encoded_data[0];
    let buffer = new Buffer(encoded_data[1], "base64");
    const json = buffer.toString("ascii");
    const data = JSON.parse(json); // ERROR Occurs Here!

    // check algorithm - not relevant to error
    if (!data.algorithm || data.algorithm.toUpperCase() != "HMAC-SHA256") {
        console.error("Unknown algorithm. Expected HMAC-SHA256");
        return null;
    }

    // check sig - not relevant to error
    const expected_sig = crypto.createHmac("sha256",secret).update(encoded_data[1]).digest("base64").replace(/\+/g,"-").replace(/\//g,"_").replace("=","");
    if (sig !== expected_sig) {
        console.error("Bad signed JSON Signature!");
        return null;
    }

    return data;
}