模仿jquery选择器 $('#id') $('.className')

function $(ss){var start=ss.substring(0,1);var obj=null;switch(start){case '#':obj=document.getElementById(ss.substring(1));break;case '.':obj=new Array();var all=document.getElementsByTagName('*');for(var i=0;i<all.length;i++){if(all[i].nodeType==1 && all[i].getAttribute('class')==ss.substring(1)){obj.push(all[i]);}}break;default:break;}if(obj!=null){if(obj.length){alert(obj.length);}else{alert(obj.innerHTML);}}}window.onload=function(){$('#book');        $('.book');}

<div id='book'>book</div><div class='book'><div class='book'><div class='book'>book</div></div></div><div class='book'><div class='book'>book</div></div><div class='book'>book</div>