事件代理

function bindEvent(elem, event, selector, fn) {
    if (fn == null) {
        fn = selector
        selector = null
    }
    elem.addEventListener(event, function(e) {
        var target
        if (selector) {
            target = e.target
            if (target.matches(selector)) {
                fn.call(target, e)
            }
        } else {
            fn(e)
        }
    })
}