angular - BehaviorSubject next not defined -

i getting runtime error when set next(..) on behaviorsubject firebase.user | null type.

subscribe.js:165 typeerror: cannot read property 'next' of undefined 

here code thats causing issue.

export class appauthservice {     private currentusersubject = new behaviorsubject<firebase.user |null>(null);    public currentuser = this.currentusersubject.asobservable().distinctuntilchanged();     private isauthenticatedsubject = new replaysubject<boolean>(1);    public isauthenticated = this.isauthenticatedsubject.asobservable();     constructor(public afauth: angularfireauth) {       afauth.auth.onauthstatechanged(this.onauthstatechanged);     }      onauthstatechanged(user: firebase.user): void {       if (user) {         this.currentusersubject.next(user);    <------err         this.isauthenticatedsubject.next(true);         console.log('loggedin!');       } else {         this.currentusersubject.next(null);   <-------err         this.isauthenticatedsubject.next(false);         console.log('gone!');       }    }  } 

the error shows anytime when this.currentusersubject.next(..) set user obj or null. suspect because currentusersubject initialized null , next of null wrong?...

private currentusersubject = new behaviorsubject<firebase.user |null>(null); 

i not able initialize new firebase.user() because interface , not object.

also declaring currentusersubject subject results in same error.

private currentusersubject = new subject();  

what best way convert firebase.user interface subject/replaysubject? trying when user changes on profile component dynamic update on firebase server.

thanks.

use fat arrow functions:

constructor(public afauth: angularfireauth) {   afauth.auth.onauthstatechanged(user => this.onauthstatechanged(user)); }