How to add a class to a DOM element without using Jquery - Angular 6

Question

Currently I have defined through bootstrap the following template of my component:

<div class="container">

  <div class="row my-4">
    <div class="col-md-12 d-flex justify-content-center">
      <h2> EVALUACIÓN DE {{ curso }} </h2>
    </div>
  </div>

  <div class="row">
    <div class="col-md-9">
      <h4>Pregunta 1 de 20 </h4>
      <p>¿Cúal de las siguientes alternativas es correcta? </p>
    </div>
    <div class="col-md-3 d-flex align-items-center" id="icono-reloj">
      <img class="mx-2" src="/assets/img/alarm-fill.svg" alt="" width="24" height="24" title="timer"> <span class="mx-3">0:20</span>
    </div>
  </div>

  <div class="row">
    <div class="col-md-12">
      <div class="list-group" ngFor="let alternativa of alternativas">
        <p class="list-group-item list-group-item-action" (click)="avanzar()">este es un ejemplo de una alternativa ...</p>
      </div>
    </div>
  </div> 
</div>

I want to implement that when the browser screen is reduced to less than 767.98px it will center the question lines.

From what I have read this would be done with jquery but it is no longer recommended. Additionally I read that the ideal would be to use the class "Renderer2" So my query would be:

That part of the documentation on Renderer2 will allow me to make the sensing of the screen size since in some way there must be a kind of media query which detects when the screen is less than 767.98px and then adds the bootstrap classes that I defined so that the divs i wish to focus on.

Answer 1

 function windowResizeFunc() { function reportWindowSize() { let h = window.innerHeight; let w = window.innerWidth; console.log("Current Width: "+ w); if( w < 600 ) { document.getElementById("yourElementId").classList.add("center"); } else { document.getElementById("yourElementId").classList.remove("center"); } } window.onresize = reportWindowSize; } windowResizeFunc();

 #yourElementId { border: 1px solid #5599cc; }.center{ text-align: center; }

 <div id="yourElementId">Some text</div>

You can use VanillaJS

Answer 2

If you're looking for an angular solution specifically:

import {
  Component,
  OnInit,
  HostListener,
  ViewChild,
  ElementRef,
} from "@angular/core";

@Component({
  selector: "app-header",
  templateUrl: "./header.component.html",
  styleUrls: ["./header.component.scss"],
})
export class HeaderComponent implements OnInit {
  currentWindowWidth: number;
  isMobile: boolean;

  @ViewChild("control") control: ElementRef;

  constructor() {}

  ngOnInit() {}

  @HostListener("window:resize", ["$event"])
  public onResize(window) {
    this.isMobile = window.currentTarget.innerWidth < 993 ? true : false;
    this.toggleMobileClasses(this.control.nativeElement, this.isMobile)
  }

  @HostListener("window:load", ["$event"])
  public onLoad(window) {
    this.isMobile = window.currentTarget.innerWidth < 993 ? true : false;
  }

  toggleMobileClasses(targetElement: HTMLElement, isMobile) {
    isMobile ? targetElement.classList.add('text-center') : targetElement.classList.toggle('text-center');
  }

}

HostListener is the key here

Decorator that declares a DOM event to listen for, and provides a handler method to run when that event occurs. Angular HostListener Docs

<div #control class="container">

  <div class="row my-4">
    <div class="col-md-12 d-flex justify-content-center">
      <h2> EVALUACIÓN DE {{ curso }} </h2>
    </div>
  </div>

  <div class="row">
    <div class="col-md-9">
      <h4>Pregunta 1 de 20 </h4>
      <p>¿Cúal de las siguientes alternativas es correcta? </p>
    </div>
    <div class="col-md-3 d-flex align-items-center" id="icono-reloj">
      <img class="mx-2" src="/assets/img/alarm-fill.svg" alt="" width="24" height="24" title="timer"> <span class="mx-3">0:20</span>
    </div>
  </div>

  <div class="row">
    <div class="col-md-12">
      <div class="list-group" ngFor="let alternativa of alternativas">
        <p class="list-group-item list-group-item-action" (click)="avanzar()">este es un ejemplo de una alternativa ...</p>
      </div>
    </div>
  </div> 
</div>

Also if you're using Angular Universal for SSR, this approach wont throw any errors about window is not defined during server parsing and populating the dynamic HTML