firebase.auth().signInWithEmailAndPassword()    .then(function(result){// User signed in. No 2nd factor challenge is needed.    })    .catch(function(error){if (error.code =='auth/multi-factor-auth-required') {var resolver = error.resolver;// Show UI to let user select second factor.var multiFactorHints = resolver.hints;      }else {// Handle other errors.      }    });// The enrolled second factors that can be used to complete// sign-in are returned in the `MultiFactorResolver.hints` list.// UI needs to be presented to allow the user to select a second factor// from that list.var selectedHint =// ; selected from multiFactorHintsvar phoneAuthProvider =new firebase.auth.PhoneAuthProvider();var phoneInfoOptions = {multiFactorHint: selectedHint,session: resolver.session};phoneAuthProvider.verifyPhoneNumber(  phoneInfoOptions,  appVerifier).then(function(verificationId){// store verificationID and show UI to let user enter verification code.});// UI to enter verification code and continue.// Continue button click handlervar phoneAuthCredential =    firebase.auth.PhoneAuthProvider.credential(verificationId, verificationCode);var multiFactorAssertion =    firebase.auth.PhoneMultiFactorGenerator.assertion(phoneAuthCredential);resolver.resolveSignIn(multiFactorAssertion)    .then(function(userCredential){// User signed in.    });