How can I add a class to an element on hover?

How to add class to a div when hovered on the div.

Template -

<div class="red">On hover add class ".yellow"</div>

Component -

import {Component} from 'angular2/core';

@Component({
  selector: 'hello-world',
  templateUrl: 'src/hello_world.html',
  styles: [`
    .red {
      background: red;
    }
    
    .yellow {
      background: yellow;
    }
  
  `]
})
export class HelloWorld {
}

Demo

[ NOTE - I specifically want to add a new class and not modify the existing classes ]

Sigh! It is such a normal use case and I do not see any straight forward solution yet!

Not to dirty the code I just coded simple hover-class directive.

<span hover-class="btn-primary" class="btn" >Test Me</span>

Running Sample

Code Editor stackblitz

Here below the directive,

import { Directive, HostListener, ElementRef, Input } from '@angular/core';

@Directive({
  selector: '[hover-class]'
})
export class HoverClassDirective {

  constructor(public elementRef:ElementRef) { }
  @Input('hover-class') hoverClass:any;  
  
  @HostListener('mouseenter') onMouseEnter() {
    this.elementRef.nativeElement.classList.add(this.hoverClass);
 }

  @HostListener('mouseleave') onMouseLeave() {
    this.elementRef.nativeElement.classList.remove(this.hoverClass);
  }
  
}

You can also just use something like:

[ngClass]="color" (mouseover)="changeStyle($event)" (mouseout)="changeStyle($event)"

Then in the component

color:string = 'red';

changeStyle($event){
  this.color = $event.type == 'mouseover' ? 'yellow' : 'red';
}

Plunker

Alternatively, do everything in the markup:

[ngClass]="color" (mouseover)="color='yellow'" (mouseout)="color='red'"

Simple as below

<button [class.btn-success]="mouseOvered" 
  (mouseover)="mouseOvered=true"
  (mouseout)="mouseOvered=false"> Hover me </button>

LIVE DEMO

In case you set the styles programmatically (e.g. from an attribute in the component) and want it to change on hover, you can have a look at [this Plunker demo][1].

It also answers the question when multiple elements have to respond to the mouseover event.

Here is the code:

@Component({
    selector: 'app',
    template: `
    <div id="1" 
      (mouseover)="showDivWithHoverStyles(1)" 
      (mouseout)="showAllDivsWithDefaultStyles()" 
      [ngStyle]="hoveredDivId ===1 ? hoveredDivStyles : defaultDivStyles">
      The first div
    </div>
    
    <div id="2" 
      (mouseover)="showDivWithHoverStyles(2)" 
      (mouseout)="showAllDivsWithDefaultStyles()" 
      [ngStyle]="hoveredDivId === 2 ? hoveredDivStyles :  defaultDivStyles">
      The second div
    </div>`
})
class App{
  hoveredDivId;
  defaultDivStyles= {height: '20px', 'background-color': 'white'};
  hoveredDivStyles= {height: '50px', 'background-color': 'lightblue'};
  
  showDivWithHoverStyles(divId: number) {
    this.hoveredDivId = divId;
  }

  showAllDivsWithDefaultStyles() {
    this.hoveredDivId = null;
  }
}

[1]: https://plnkr.co/edit/aNAL9z?p=preview "Angular 2+ change element style programmatically on hover"

<li *ngFor="let q of questions;let i = index" class="list-group-item"  (mouseenter)="isHover = i" [ngClass]="{'active' : isHover === i}">
            <h5>Question 1</h5>
            <p>what is the largest man made structure on the earth ?</p>
        </li>

isHover and questions are properties declare in the .ts file.

I took Davut's example and changed it to handle multiple classes...

import {Directive, HostListener, ElementRef, Input} from '@angular/core';

@Directive( {
	selector: '[hover-class]'
} )
export class HoverClassDirective {

	constructor( public elementRef: ElementRef ) {
	}

	@Input( 'hover-class' ) hoverClass: any;

	@HostListener( 'mouseenter' ) onMouseEnter() {
		this.update( 'add' );
	}

	@HostListener( 'mouseleave' ) onMouseLeave() {
		this.update( 'remove' );
	}

	protected update( action: string ): void {
		this.hoverClass.split( ' ' ).forEach( item => this.elementRef.nativeElement.classList[action]( item ) );
	}
}

@HostListener decorator is also a good option if you are applying on entire component.

keep the html as it is and in the component add @HostListener

  <div class="red">On hover add class ".yellow"</div> 

  @HostListener('mouseenter') onMouseEnter() {
    this.elementRef.nativeElement.class = 'red';
  }

  @HostListener('mouseleave') onMouseLeave() {
    this.elementRef.nativeElement.class = 'yellow';
  }

Shortest solution - HTML only

<span #newTask class="btn"
      [ngClass]="newTask.matches(':hover') ? 'btn-success': 'btn-secondary'"
      (mouseover)="''" (mouseleave)="''"
>
  New
</span>