JavaScript怎样获取一个元素的样式信息

function getStyle(elem, cssprop, cssprop2){
 if(elem.currentStyle){
  return elem.currentStyle[cssprop]
  }else if(document.defaultView && document.defaultView.getComputedStyle){
  return document.defaultView.getComputedStyle(elem, "").getPropertyValue(cssprop2)
 }else{
  return null
 }
}

<style> 
 #elem{font-size:23px;} 
</style> 
<input id ="elem" style="font-size:18px;"/> 
<script>
elem.style.fontSize = '12px'; 
console.log(getStyle(elem, "fontSize", "font-size")) 
// 输出12px 
</script>