class MainClass {
    constructor() {
        this.playButton = new ShowButton(this.display);
        this.name = "some stuff";
    }

    display() {
        console.log("calling display()");
        console.log(this.name);
    }
}

class ShowButton {
    constructor(aPlayFunction) {
    this.x = 100;
    this.y = 100;
    this.button = document.createElement('playButton');
    this.button.innerHTML = 'play';
    this.button.onclick = aPlayFunction;
    document.body.appendChild(this.button);
    }
}
let main = new MainClass();