title: check if user is already exists in firebase
body: <p>I started with firebase (web) I would like to know how to programmatically verify that a user already exists in the firebase user table without using the</p>
  
  <pre><code>firebase.auth().signInWithEmailAndPassword(email, password).then(...)
  </code></pre>
  
  <p>I would like to know if there is not another method to directly verify if the user does not exist.
  thank..</p>