Angular 6 testing mock dependency from class

I am new to Angular and I have started to write my first test. Could someone help me understand how the dependency injection works ?

I have a class called Level.ts which looks something like this:

export class Level {
 Title: string,
 isTitle: boolean
}

and my actual component called Previewer.component.ts which looks like

import { Input,Component, OnInit } from '@angular/core';
import { Level} from '../Level';

@Component({
  selector: 'app-previewer',
  templateUrl: './previewer.component.html',
  styleUrls: ['./previewer.component.css']
})
export class Previewer implements OnInit {
  @Input() level: Level;

confirmTitle(){
 this.level.Title = "Level 1";
 this.level.isTitle = true;
}
}

I must add that I am not using a service currently and would want to test the previewer.component.ts file. However when I run the test, it throws an error saying that 'TypeError: Cannot read property 'isTitle' of undefined'. I have also used the NO_ERRORS_SCHEMA, but still I get this error thown. Kindly help