js 如何获取class的元素 以及创建方法getElementsByClassName

function getElementsByClassName(n) { 
var classElements = [],allElements = document.getElementsByTagName('*'); 
for (var i=0; i< allElements.length; i++ ) 
{ 
if (allElements[i].className == n ) { 
classElements[classElements.length] = allElements[i]; 
} 
} 
return classElements; 
}

var redClassElements = getElementsByClassName('aa'); 
for (var i=0; i<redClassElements.length; i++) { 
      //do something
}