通过冒泡事件 实现点击按钮弹出层，点击页面其它地方隐藏层

document.onclick = function (e) {

    if(e.target.tagName == "INPUT" && e.target.id == "btn") {

        var div1 = document.getElementById("div_1");
        //alert(div1.style.display);
        if(div1.style.display == "block"){
            div1.style.display = "none"
        } else {
            div1.style.display = "block"
        }
    } else {
        document.getElementById("div_1").style.display = "none"
    }
}