JavaScript中的this指针与回调函数

class A {
    setEventListener(func) {
        this.callback = func;
    }

    triggerEvent() {
        this.callback();
    }
 }

class B {
    constructor(a) {
        a.setEventListener(this.onEvent);
        this.localValue = "nihao";
    }

    onEvent() {
        console.log(this.localValue);
        // console.log(this);
    }
}


var a = new A();
var b = new B(a);

a.triggerEvent();

A { callback: [Function: onEvent] }

a.setEventListener(this.onEvent);

a.setEventListener(this.onEvent.bind(this));