unable to initialize class property inside ngOnit error

Question

I'm getting an error that says monetization has no inializer. I'm also getting an error inside ngOninit that says monetization is not assignable to type string | null.

export class MoviesPageComponent implements OnInit {
  movies: MovieResults[] = [];
  monetization: string;
  constructor(private getMovies: GetMoviesService,
              private route: ActivatedRoute) { }
  ngOnInit(): void {
    this.route.paramMap.subscribe((params: ParamMap) => {
      this.monetization = params.get('monetization');
    });

score 1 · Accepted Answer · answered Mar 14 '22 at 17:45

You get this error because monetization is not initialized in the constructor.

What you'll need to do, is make the property optional.

export class MoviesPageComponent implements OnInit {
  movies: MovieResults[] = [];
  monetization?: string; // <-- add ? here
  constructor(private getMovies: GetMoviesService,
              private route: ActivatedRoute) { }
  ngOnInit(): void {
    this.route.paramMap.subscribe((params: ParamMap) => {
      this.monetization = params.get('monetization');
    });

score 0 · Answer 2 · answered Mar 14 '22 at 17:57

If we look at the function specification of ParamMap.get...

get(name: string): string | null

the return type is string | null, however type of monetization property is

monetization: string;

and the assignment fails due to type inconsistency when we are doing

 this.monetization = params.get('monetization'); //<-- Return type is string | null

To fix this,

Make the property optional, as stated in the other answer. Like monetization?: string. Suffixing the property name with a ? implicits an undefined in the type
Or, provide a default to satisfy null-ness of the value after reading from ParamMap.get(...). Like this.monetization = params.get('monetization') ?? '';
If uncomfortable with making property optional, you can explicitly provide string | undefined | null to the type of the property. Like monetization: string | null | undefined.

unable to initialize class property inside ngOnit error

2 Answers2