How can I change a body tag class in Angular 10 (best practice)?

Question

I want to switch between two classes (light and dark) at TAG Body.

What I did? I created a service:

import { Injectable } from '@angular/core';

@Injectable({
  providedIn: 'root'
})
export class ThemeService {
  body = document.body;

  constructor() { }

  changeLight() {
    this.body.classList.replace('light', 'dark');
  }

  changeDark() {
    this.body.classList.replace('dark', 'light');
  }
}

It is working as expected but I know that this code does not use best practices.
What is the correct way to change between these two classes?

Answer 1

Edit: Added a service to the stackblitz, but again, there are many ways to do this. This is just a starting point.

While the "right way" is subjective, you have some options to make it "Angular-y"

Component:

import { Component, Inject } from '@angular/core';
import { DOCUMENT } from '@angular/common';

// Create a type that accepts either the string 'light' or 'dark' only
type Theme = 'light' | 'dark';

@Component({
  selector: 'my-app',
  templateUrl: './app.component.html',
  styleUrls: [ './app.component.css' ]
})
export class AppComponent  {
  // Default to 'light' theme
  currentTheme: Theme = 'light';

  // Inject document which is safe when used with server-side rendering
  constructor(@Inject(DOCUMENT) private document: Document) {
    // Add the current (light) theme as a default
    this.document.body.classList.add(this.currentTheme);
  }

  // Swap them out, and keep track of the new theme
  switchTheme(newTheme: Theme): void {
    this.document.body.classList.replace(this.currentTheme, newTheme)
    this.currentTheme = newTheme;
  }
}

HTML:

<p>
  Current theme: {{ currentTheme }}

  <button (click)="switchTheme('light')">Light mode</button>
  <button (click)="switchTheme('dark')">Dark mode</button>
</p>

Many ways to do this, but one benefit of defining the types is if you provide a bad value, such as:

<p>
  Current theme: {{ currentTheme }}

  <button (click)="switchTheme('light')">Light mode</button>
  <button (click)="switchTheme('dark')">Dark mode</button>
  <button (click)="switchTheme('noop')">Invalid</button>
</p>

You'll get an error:

Argument of type '"noop"' is not assignable to parameter of type 'Theme'.

StackBlitz