How to declare types for properties and how to call methods from the html files

app.component.ts

import { BrowserModule } from '@angular/platform-browser';
import { NgModule } from '@angular/core';

import { AppComponent } from './app.component';
import { UserComponet } from './components/user/user.component';

@NgModule({
  declarations: [
    AppComponent,
    UserComponet
  ],
  imports: [
    BrowserModule
  ],
  providers: [],
  bootstrap: [AppComponent]
})
export class AppModule { }

user.component.html

<!-- <h2>John Doe</h2> -->
<!-- String interpolation -->
<h2>{{ firstName }} {{ lastName }}</h2>
<ul>
  <li>Age: {{ showAge() }}</li>
  <li>Address: {{ address.street }}, {{ address.city }}, {{ address.country }}</li>
</ul> 

user.component.ts

import { Component } from "@angular/core";
@Component({
  selector: "app-user",
  templateUrl: "./user.component.html",
  styleUrls: ["./user.component.css"]
})
export class UserComponet {
  firstName : string; // using types and calling from constructor 
  lastName : string;
  age : number;
  address;

  // Methods
  constructor() {
    // Using Types
    this.firstName = 'Nasirul';
    this.lastName = 'Islam';
    this.age = 24;

    this.address = {
      street: '214/B',
      city: 'Dhaka',
      country: 'Bangladesh'
    }
  }
  // we can also use methods for string interpolation 
  showAge() {
    return this.age+2;
  }
}