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

As the error says, localStorage.getItem() can return either a string or null. JSON.parse() requires a string, so you should test the result of localStorage.getItem() before you try to use it.

For example:

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

or perhaps:

const userJson = localStorage.getItem('currentUser');
this.currentUser = userJson !== null ? JSON.parse(userJson) : new User();

See also the answer from Willem De Nys. If you are confident that the localStorage.getItem() call can never return null you can use the non-null assertion operator to tell typescript that you know what you are doing:

this.currentUser = JSON.parse(localStorage.getItem('currentUser')!);