how to read firebase database in javascript code example

Example 1: javascript write to firebase

function writeUserData(userId, name, email, imageUrl) {
  firebase.database().ref('users/' + userId).set({
    username: name,
    email: email,
    profile_picture : imageUrl
  });
}

Example 2: write firebase data using variables javascript

foo = {}; 
foo[variable] = 'more stuff'; 
fb.set(foo);