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

Question

I am very beginner in typescript. I am trying to get my local storage variable auth value. I know variable in local storage are stored as a string. So I am converting it into Boolean using JSON.parse But I am getting error saying [Argument of type 'string | null' is not assignable to parameter of type 'string'. Type 'null' is not assignable to type 'string'].

Getting error at line: 2 where I am declaring auth Variable

let leftmenu;
    const auth:boolean = JSON.parse(localStorage.getItem('auth'));
    if (auth === true) {
        leftmenu = (
            <React.Fragment>
                <Navbar.Text>
                    Signed in as: <a href="#login">Mark Otto</a>
                </Navbar.Text>
                <Button variant="outline-success">Logout</Button>
            </React.Fragment>);
    } else {
        leftmenu = (
            <React.Fragment>
                <Button variant="outline-success">Login</Button>
                <Button variant="outline-success">Sign Up</Button>
            </React.Fragment>
        )
    }

Answer 1

Because it's possible that localStorage.getItem('auth') will return null, while JSON.parse requires a string.

You will need to do a null check before parsing the variable.

cosnt authRaw: string = localStorage.getItem('auth');
if(authRaw !== null){
    const auth: boolean = JSON.parse(authRaw);
}

A simpler approach is to use ?? to add a fallback value as an alternative to localStorage.getItem('auth')

const auth:boolean = JSON.parse(localStorage.getItem('auth') ?? "false");