[css] How and where to use ::ng-deep?

I'm new to Angular 4, so could anyone please explain how and where to use ::ng-deep in Angular 4?

Actually I want to overwrite some of the CSS properties of the child components from the parent components. Moreover is it supported on IE11?

Thanks for the help.

USAGE

::ng-deep, >>> and /deep/ disable view encapsulation for specific CSS rules, in other words, it gives you access to DOM elements, which are not in your component's HTML. For example, if you're using Angular Material (or any other third-party library like this), some generated elements are outside of your component's area (such as dialog) and you can't access those elements directly or using a regular CSS way. If you want to change the styles of those elements, you can use one of those three things, for example:

::ng-deep .mat-dialog {
  /* styles here */
}

For now Angular team recommends making "deep" manipulations only with EMULATED view encapsulation.

DEPRECATION

"deep" manipulations are actually deprecated too, BUT it stills working for now, because Angular does pre-processing support (don't rush to refuse ::ng-deep today, take a look at deprecation practices first).

Anyway, before following this way, I recommend you to take a look at disabling view encapsulation approach (which is not ideal too, it allows your styles to leak into other components), but in some cases, it's a better way. If you decided to disable view encapsulation, it's strongly recommended to use specific classes to avoid CSS rules intersection, and finally, avoid a mess in your stylesheets. It's really easy to disable right in the component's .ts file:

@Component({
  selector: '',
  template: '',
  styles: [''],
  encapsulation: ViewEncapsulation.None  // Use to disable CSS Encapsulation for this component
})

You can find more info about the view encapsulation in this article.

Just an update:

You should use ::ng-deep instead of /deep/ which seems to be deprecated.

Per documentation:

The shadow-piercing descendant combinator is deprecated and support is being removed from major browsers and tools. As such we plan to drop support in Angular (for all 3 of /deep/, >>> and ::ng-deep). Until then ::ng-deep should be preferred for a broader compatibility with the tools.

You can find it here

Make sure not to miss the explanation of :host-context which is directly above ::ng-deep in the angular guide : https://angular.io/guide/component-styles. I missed it up until now and wish I'd seen it sooner.

::ng-deep is often necessary when you didn't write the component and don't have access to its source, but :host-context can be a very useful option when you do.

For example I have a black <h1> header inside a component I designed, and I want the ability to change it to white when it's displayed on a dark themed background.

If I didn't have access to the source I may have to do this in the css for the parent:

.theme-dark widget-box ::ng-deep h1 { color: white; }

But instead with :host-context you can do this inside the component.

 h1 
 {
     color: black;       // default color

     :host-context(.theme-dark) &
     {
         color: white;   // color for dark-theme
     }

     // OR set an attribute 'outside' with [attr.theme]="'dark'"

     :host-context([theme='dark']) &
     {
         color: white;   // color for dark-theme
     }
 }

This will look anywhere in the component chain for .theme-dark and apply the css to the h1 if found. This is a good alternative to relying too much on ::ng-deep which while often necessary is somewhat of an anti-pattern.

In this case the & is replaced by the h1 (that's how sass/scss works) so you can define your 'normal' and themed/alternative css right next to each other which is very handy.

Be careful to get the correct number of :. For ::ng-deep there are two and for :host-context only one.

I would emphasize the importance of limiting the ::ng-deep to only children of a component by requiring the parent to be an encapsulated css class.

For this to work it's important to use the ::ng-deep after the parent, not before otherwise it would apply to all the classes with the same name the moment the component is loaded.

Using the :host keyword before ::ng-deep will handle this automatically:

:host ::ng-deep .mat-checkbox-layout

Alternatively you can achieve the same behavior by adding a component scoped CSS class before the ::ng-deep keyword:

.my-component ::ng-deep .mat-checkbox-layout {
    background-color: aqua;
}

Component template:

<h1 class="my-component">
    <mat-checkbox ....></mat-checkbox>
</h1>

Resulting (Angular generated) css will then include the uniquely generated name and apply only to its own component instance:

.my-component[_ngcontent-c1] .mat-checkbox-layout {
    background-color: aqua;
}

Use ::ng-deep with caution. I used it throughout my app to set the material design toolbar color to different colors throughout my app only to find that when the app was in testing the toolbar colors step on each other. Come to find out it is because these styles becomes global, see this article Here is a working code solution that doesn't bleed into other components.

<mat-toolbar #subbar>
...
</mat-toolbar>

export class BypartSubBarComponent implements AfterViewInit {
  @ViewChild('subbar', { static: false }) subbar: MatToolbar;
  constructor(
    private renderer: Renderer2) { }
  ngAfterViewInit() {
    this.renderer.setStyle(
      this.subbar._elementRef.nativeElement, 'backgroundColor', 'red');
  }

}