<button @click="increment1">+1</button>
<button @click="increment2($event)">+2</button>
<button @click="increment3(3, $event)">+3</button>
<button @click="increment4(4)">+4</button>

increment1(event) {
    // eslint-disable-next-line
    console.log('event1', event, event.__proto__.constructor)
    this.num++
},
increment2(event) {
    // eslint-disable-next-line
    console.log('event2 target', event.target)
},
increment3(val, event) {
    // eslint-disable-next-line
    console.log('event3 target', event.target)
    this.num = this.num + val
},
increment4(val) {
    // eslint-disable-next-line
    console.log('event4 target', event.target)
    this.num = this.num + val
},

请问下。

1. 对比 increment1 和 increment2，调用时无论传参与否，为何都能访问到 event。
2. 对比 increment3 和 increment4，传不传 $event 也都能访问到 event。 $event 有什么用呢。