login.js

var tempErrorCreds;
var tempProviderName;

function retrieveIdToken(successCallback, errorCallback) {
	if(firebase.auth().currentUser === null){
		if(errorCallback !== null) 
			errorCallback("User is null");
		return;
	}

	firebase.auth().currentUser.getIdToken().then(function (idToken) {
		var resultObj = {
			token: idToken,
			displayName: firebase.auth().currentUser.displayName
		};

		if (successCallback !== undefined) {

			successCallback(resultObj);
		}
	})
		.catch(function (error) {
			console.log(error);
			if (errorCallback !== undefined)
				errorCallback(error.message);
		});
}

function anonymousLogin(successCallback, errorCallback) {
	var resultObj = {
		token: "",
		displayName: "guest"
	};

	if (successCallback !== undefined) {

		successCallback(resultObj);
	}
}

function firebaseLogin(providerName, successCallback, errorCallback) {
	if (providerName === "anonymous") {
		anonymousLogin(successCallback, errorCallback);
		return;
	}

	var user = firebase.auth().currentUser;

	if(user != null && !user.isAnonymous){
		retrieveIdToken(successCallback, errorCallback);
		return;
	}

	var provider = getProvider(providerName);
	firebase.auth().useDeviceLanguage();

	//var task = firebase.auth().currentUser.isAnonymous ? firebase.auth().signInWithPopup(provider) : firebase.auth().linkWithPopup(provider);

	firebase.auth().signInWithPopup(provider).then(function (result) {
			console.log("Successful sign in");
			retrieveIdToken(successCallback, errorCallback);
		})
		.catch(function (error) {
			// Handle Errors here.
			var errorCode = error.code;
			var errorMessage = error.message;
			// The email of the user's account used.
			var email = error.email;
			// The firebase.auth.AuthCredential type that was used.
			tempErrorCreds = error.credential;
			console.log(error);

			if (errorCallback !== undefined)
				errorCallback(error.message);

			if (errorCode === 'auth/account-exists-with-different-credential') {
				// User's email already exists.
				// Get sign-in methods for this email.
				firebase.auth().fetchSignInMethodsForEmail(email).then(function (methods) {
					// the first method in the list will be the "recommended" method to use.
					if (methods.length == 0)
						return;
					// Sign in to provider.
					tempProviderName = methods[0].trim();
					setModalContent("generalModalContent",
						"<div id =\"continueWindow\"><span class=\"close\" id=\"closeButton\" onclick=\"hideModal('generalModal')\">&times;</span><p>Please press the button to login: </p><button onclick=\"continueLogin()\">Continue Login</button></div>");
					showModal("generalModal");
				});
			}
		});
}

function firebaseLogout() {
	firebase.auth().signOut().catch(function (error) {
		console.log(error);
	});
}

function getCurrentUserDisplayName() {
	var user = firebase.auth().currentUser;
	var displayName = "";
	if (user) {
		displayName = user.displayName;
	}
	return displayName;
}

function getProvider(providerName) {
	if (providerName && providerName.indexOf("facebook") != -1)
		return new firebase.auth.FacebookAuthProvider()
	else
		return new firebase.auth.GoogleAuthProvider()
}

function setModalContent(modalContentId, contentString) {
	content = document.getElementById(modalContentId);
	if (content) {
		content.innerHTML = contentString;
	}

}

function continueLogin() {
	hideModal("generalModal");
	var provider = getProvider(tempProviderName);
	firebase.auth().signInWithPopup(provider).then(
		function (result) {
			if (!tempErrorCreds) {
				return;
			}
			// As we have access to the pending credential, we can directly call the link method.
			result.user.linkAndRetrieveDataWithCredential(tempErrorCreds).then(function (usercred) {
				//goToApp();
			});
		});

}

function showModal(modalId) {
	modal = document.getElementById(modalId);
	if (modal)
		modal.style.display = "block";
}

function hideModal(modalId) {
	modal = document.getElementById(modalId);
	if (modal)
		modal.style.display = "none";
}