实现getElementsByClassName

var getElementsByClassName = function (searchClass, node,tag) { 
  if(document.getElementsByClassName){ //原生
    var nodes =  (node || document).getElementsByClassName(searchClass),result = [];
      for(var i=0 ;node = nodes[i++];){//多个class检索
        if(tag !== "*" && node.tagName === tag.toUpperCase()){
          result.push(node)
        }
      }
      return result
    }else{
      node = node || document;
      tag = tag || "*";
      var classes = searchClass.split(" "),
      elements = (tag === "*" && node.all)? node.all : node.getElementsByTagName(tag),
      patterns = [],
      current,
      match;
      var i = classes.length;
      while(--i >= 0){
        patterns.push(new RegExp("(^|\s)" + classes[i] + "(\s|$)"));
      }
      var j = elements.length;
      while(--j >= 0){
        current = elements[j];
        match = false;
        for(var k=0, kl=patterns.length; k<kl; k++){
          match = patterns[k].test(current.className);
          if (!match)  break;
        }
        if (match)  result.push(current);
      }
      return result;
    }
  }