0级监听(在标签内写onclick事件)：
<a href="javascript:void(0);" target="_blank" rel="external nofollow" οnclick="js_method()"></a>
<a href="javascript:;" target="_blank" rel="external nofollow" οnclick="js_method()"></a>
<a href="#" target="_blank" rel="external nofollow" οnclick="js_method();return false;"></a>
<button οnclick="copyText()">复制文本</button>
1级监听（在JS写οnclick=function（）{}函数）：
var btn=$('#hash');
btn.οnclick=function(){
alert('');
};
2级监听：
document.getElementById("myBtn").addEventListener("click", function(){
document.getElementById("demo").innerHTML = "Hello World";
});