How to toggle Menu in Ionic 4

Simply encapsulate your ion-button within an ion-menu-toggle element, like so:

<ion-menu-toggle>
  <ion-button>Toggle Menu</ion-button>
</ion-menu-toggle>

View the documentation here

EDIT: If you don't want to use ion-menu-toggle, you can do this instead:

In your app.component.ts:

import { MenuController } from '@ionic/angular'; //import MenuController to access toggle() method.

@Component({
  selector: 'app-root',
  templateUrl: 'app.component.html'
})
export class AppComponent {
  constructor(
    public menuCtrl: MenuController
  ) {
    this.initializeApp();
  }

toggleMenu() {
    this.menuCtrl.toggle(); //Add this method to your button click function
  }

}

And in your app.component.html:

<ion-button (click)="toggleMenu()">Toggle Menu</ion-button>

To view all methods, check the docs here.