手写一个简单的事件绑定函数

function bindEvent(elem,type,selector,fn){
    if (fn == null) {
        fn = selector
        selector = null
    }


    elem.addEventListener(type,event=>{
        const target = event.target
        if (selector) {
            // 代理绑定
            if(target.matches(selector)){
                fn.call(target, event)
            }
        } else {
            // 普通绑定
            fn.call(target, event)
        }
    })
}