Argument of type 'string | null' is not assignable to parameter of type 'string | undefined'

The subject exists here also, but I’m still stuck.

I have an error in the isAuth() method:

Error: src/app/services/auth.service.ts:22:38 - error TS2345: Argument of type 'string 
| null' is not assignable to parameter of type 'string | undefined'.

Type 'null' is not assignable to type 'string | undefined'.

22     if(this.jwtHelper.isTokenExpired(token) || !localStorage.getItem('token')){  

I don’t understand what I have to change?

isAuth():boolean{
    const token = localStorage.getItem('token');
    if(this.jwtHelper.isTokenExpired(token) || !localStorage.getItem('token')){
      return false;
    }
    return true;
}

code

>Solution :

the value of token is nullable because if there is no value for the key ‘token’ in your local storage, then localStorage returns null.
so what you can do is:
first check if there is a value for token or not.

isAuth():boolean{
    const token = localStorage.getItem('token');
    if((token && this.jwtHelper.isTokenExpired(token)) || !localStorage.getItem('token')){
      return false;
    }
    return true;
}

but the better approach is this:

isAuth():boolean{
    const token = localStorage.getItem('token');
    return token && !this.jwtHelper.isTokenExpired(token);
}