In google firebase auth method, I got a promise object back in userRef, in the PromiseResult, there is a function called onSnapshot, but when I call that function, I got error Uncaught (in promise) TypeError: userRef.onSnapshot is not a function

This is the code to print out the object and calling the function.

    console.log(userRef);
    userRef.onSnapshot((snapshot) => {
      console.log("in snapsot");
    });

the screenshot of the object structure for the userRef

Any suggestion? Thanks.