firebase.auth().onAuthStateChanged(currentUser => {
  if (currentUser) {
    // User is signed in.
  } else {
    // No user is signed in.
  }
});

firebase.auth().createUserWithEmailAndPassword(email, password).catch(function(error) {
  // Handle Errors here.
  var errorCode = error.code;
  var errorMessage = error.message;
  // ...
});

firebase.auth().signInWithEmailAndPassword(email, password).catch(function(error) {
  // Handle Errors here.
  var errorCode = error.code;
  var errorMessage = error.message;
  // ...
});

var provider = new firebase.auth.GoogleAuthProvider();

var provider = new firebase.auth.FacebookAuthProvider();

var provider = new firebase.auth.TwitterAuthProvider();

var provider = new firebase.auth.GithubAuthProvider();

firebase.auth().signInWithPopup(provider);

firebase.auth().signInWithRedirect(provider);

window.recaptchaVerifier = new firebase.auth.RecaptchaVerifier('sign-in-button', {
  'size': 'invisible',
  'callback': function(response) {
    // reCAPTCHA solved, allow signInWithPhoneNumber.
    onSignInSubmit();
  }
});

var phoneNumber = getPhoneNumberFromUserInput();
var appVerifier = window.recaptchaVerifier;
firebase.auth().signInWithPhoneNumber(phoneNumber, appVerifier)
  .then(function (confirmationResult) {
    // SMS sent. Prompt user to type the code from the message, then sign the
    // user in with confirmationResult.confirm(code).
    window.confirmationResult = confirmationResult;
  }).catch(function (error) {
    // Error; SMS not sent
    // ...
  });

var code = getCodeFromUserInput();
confirmationResult.confirm(code).then(function (result) {
  // User signed in successfully.
  var user = result.user;
  // ...
}).catch(function (error) {
  // User couldn't sign in (bad verification code?)
  // ...
});