demo.ts

import { Component } from '@angular/core';

@Component({
  selector: 'my-app',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.css']
})
export class AppComponent {
  name = 'Angular';
  data = {
    firstname: 'tony'
  };

  onClick() {
    this.data.firstname = 'hudson'; // this not work
    //below work
    this.data = {
      firstname: 'hudson'
    }
  }
}