How to apply css class to a component element when it's created by router-outlet?

I have DOM that looks something like this:

<app>
    <router-outlet></router-outlet>
    <project>...</project>
</app>

where project element is inserted by the router.

How do I add a class to this element?

Assuming you always want the class applied to this component, you can use host in your component metadata:

@Component({
  selector:'project',
  host: {
      class:'classYouWantApplied'
  }
})

Resulting in:

<app>
    <router-outlet></router-outlet>
    <project class="classYouWantApplied">...</project>
</app>

use the adjacent sibling selector and the * wildcard to select the element immediately following the router-outlet

styles.css

router-outlet + * {
  /* your css */
}

The key is /deep/ keyword:

    :host /deep/ router-outlet + project {
        display: block;
        border: 10px solid black;
    }

This works without any extra configurations.

:host /deep/ router-outlet + * for whatever component dynamically created by Angular Router.

Edited 2018/3/5:

Since Angular 4.3.0 made /deep/ deprecated, its suggested alternative is ::ng-deep. And there were a long discussion about this.

You can use the adjacent sibling selector

router-outlet + project { ... }

https://developer.mozilla.org/en/docs/Web/CSS/Adjacent_sibling_selectors

but only if @drewmoore's approach doesn't apply.

You can do this with a HostBinding, which is effectively the same as using the host property that has already been mentioned, although that method throws a TSLint error with default listing rules.

In your component on which you want to apply a class:

import { Component, HostBinding, Host (optional for typing) } from '@angular/core';

@Component({...})
export class GiveMeAClassComponent {
    @HostBinding('class.some-class') someClass: Host = true;
    ...
}

And then in your root styles.scss file, you can add the following:

.some-class {
    // Styles in here will now be applied to your GiveMeAClassComponent at a root level
}

<app>
  <div class="your css class">
   <router-outlet></router-outlet>
</div>
</app>

This works for me

If you need to add a class conditionally, you can add it programmatically from the component:

constructor(private renderer: Renderer2, private elemRef: ElementRef) {
  if(someCondition){
    renderer.addClass(elemRef.nativeElement, 'myClass');
  }
}

since router injects the component after the the router-outler element, if we would like to style all injected component with the same set of rules the folowing rule is can be helpful.

the css "+" operator select the first sibling element of a certain type, while asterisk (*) is used as a wild card to select any 1st sibling of router-outlet

router-outlet + * {
  // your rules
}

Currently, Angular 6 recommends me to use @HostBindings and @HostListeners instead of the host property:

export class ProjectComponent {
  @HostBinding('class') classes = 'classYouWantApplied';
}

I created a RouterOutletHelperDirective which can be modified as necessary.

Your use-case may be different but for me :

• I needed to set a default set of classes on each router-outlet generated item
• I needed to prevent this default based on certain conditions, such as ActivatedRoute data.

You use it like this (the class is optional):

<router-outlet routerOutletHelper
               [routerOutletHelperClass]="'blue-border'"></router-outlet>

Then create the directive, add and export it to your app module.

import { Directive, ElementRef, Renderer2, Input } from "@angular/core";
import { RouterOutlet } from "@angular/router";
import { Subscription } from "rxjs";

@Directive({
    selector: 'router-outlet[routerOutletHelper]'
})
export class RouterOutletHelperDirective
{
    constructor(private routerOutlet: RouterOutlet,
                private element: ElementRef<HTMLElement>,
                private renderer: Renderer2) { }

    subscription = new Subscription();

    @Input('routerOutletHelperClass')
    customClassName: string | undefined;

    ngOnInit() 
    {
        this.subscription.add(this.routerOutlet.activateEvents.subscribe((_evt: any) => {

            // find the component element that was just added
            const componentElement = this.element.nativeElement.nextSibling;

            // add a custom class
            if (this.customClassName) 
            {
                this.renderer.addClass(componentElement, this.customClassName);
            }

            // add my default classes, unless the activated route data 
            // (specified in module routing file) has { addDefaultClasses: false }
            if (this.routerOutlet.activatedRouteData && this.routerOutlet.activatedRouteData.addDefaultClasses !== false)
            {
                // these are my application's default classes (material / theming)
                // (an additional data parameter could be 'darkTheme: boolean')
                this.renderer.addClass(componentElement, 'mat-typography');
                this.renderer.addClass(componentElement, 'rr-theme-light');
            }
        }));
    }

    ngOnDestroy()
    {    
        this.subscription.unsubscribe();
    }
}

For me it helped to wrap the router-outlet into another container.

<div class="classYouWantApplied">
  <router-outlet>
</div>

Like this you could apply the class to the surrounding container.

Add host class name which will add a class to the component then use adjacent to target the element.

@Component({
  selector:'project',
  host: {
      class:'Project-wrapper'
  }
})

now use CSS adjacent with angular

::ng-deep to target it:
::ng-deep .Project-wrapper {}

it's simple, let's say in your app component you have a
<router-outlet ></router-outlet>

inside this router-outlet you have a route component called <app-product-detail></app-product-detail>

and you want to change the view of in the app.component.html.

first inside the the component <app-product-detail> add this snippet:

@Component({


selector: 'app-product-detail',
  host: {
    class:'Project-wrapper'
},
  templateUrl: './product-detail.component.html',
  styleUrls: ['./product-detail.component.css']
})```

class= 'could be any name'

inside app.component.css you add this snippet below:

    ::ng-deep .Project-wrapper { 
width: 85%;
        background-color: aqua;
    }