Type 'string | undefined' is not assignable to type 'string'. Type 'undefined' is not assignable to type 'string'. code example

Example 1: Argument of type 'string | null' is not assignable to parameter of type 'string'. Type 'null' is not assignable to type 'string'

this.currentUser = JSON.parse(localStorage.getItem('currentUser') || '{}');

Example 2: Type 'string | undefined' is not assignable to type 'string'.

let name1:string = person.name as string;

Example 3: Type 'string | undefined' is not assignable to type 'string'.

let name1:string = person.name!; 
//                            ^ note the exclamation mark here