getFirebaseUser.js

const admin = require("admin");
function getFirebaseUser(req, res, next) {
  console.log("Check if request is authorized with Firebase ID token");

  if (
    !req.headers.authorization ||
    !req.headers.authorization.startsWith("Bearer ")
  ) {
    console.error(
      "No Firebase ID token was passed as a Bearer token in the Authorization header.",
      "Make sure you authorize your request by providing the following HTTP header:",
      "Authorization: Bearer <Firebase ID Token>"
    );
    res.status(403).send("Unauthorized");
    return;
  }

  let idToken;
  if (
    req.headers.authorization &&
    req.headers.authorization.startsWith("Bearer ")
  ) {
    console.log("Found 'Authorization' header");
    idToken = req.headers.authorization.split("Bearer ")[1];
  }

  admin
    .auth()
    .verifyIdToken(idToken)
    .then(decodedIdToken => {
      console.log("ID Token correctly decoded", decodedIdToken);
      req.user = decodedIdToken;
      next();
    })
    .catch(error => {
      console.error("Error while verifying Firebase ID token:", error);
      res.status(403).send("Unauthorized");
    });
}

The second "if" is useless since the second condition is complementary to the first one: !(!x || !y) == !(!(x && y)) == x && y

const admin = require("firebase-admin");
instead of :
const admin = require("admin");