The argument provided is of type 'string | null' which cannot be assigned to a parameter expecting only 'string' type. The value of 'null' is not compatible with the type 'string' in Angular 12

When trying to parse the stored session data using JSON.parse(sessionStorage.getItem('owner')), we may encounter an error stating: Argument of type 'string | null' is not assignable to parameter of type 'string'. This is because the function expects a string but receives a value that is potentially null, which is not compatible with a string type.

If the code returns null when accessing the owner, it triggers another issue where null cannot be assigned to a variable of type 'string', leading to a type mismatch error.

public get owner(): Owner {
if (this._owner != null) {
  return this._owner;
} else if (this._owner == null && sessionStorage.getItem('owner') != null) {
  this._owner = JSON.parse(sessionStorage.getItem('owner')) as Owner; // Check this line for potential errors.
  return this._owner;
}
return new Owner();

}

public get token(): string {
if (this._token != null) {
  return this._token;
} else if (this._token == null && sessionStorage.getItem('token') != null) {
  this._token = sessionStorage.getItem('token') as string;
  return this._token;
}
return null;   // Be cautious here as returning null might lead to a type mismatch error.

}

Answer №1

Initially, you have defined token() to only return a string, but are attempting to return null. A possible solution is to update the definition to allow for string | null:

public get token(): string | null {
    if (this._token != null) {
        return this._token;
    } else if (this._token == null && sessionStorage.getItem('token') != null) {
        this._token = sessionStorage.getItem('token') as string;
        return this._token;
    }
    return null;
}

It is essential for any code utilizing this getter to handle this change accordingly.

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