function getStyle(elem, styleName){
	if(elem.style[styleName]){ //獲取內聯樣式
		return elem.style[styleName];
	}
	else if(elem.currentStyle){ //IE特有的屬性
		return elem.currentStyle[styleName];
	}
	else if(window.getComputedStyle){ //DOM標準屬性
		return window.getComputedStyle(elem, null)[styleName];
	}
	else{ 
		return null;
	}
}
 

考慮到函式中有重複工作的部分，優化一下：

function getStyle(elem, styleName){
	if(elem.currentStyle) {
		getStyle = function(elem, styleName) {
			if(elem.style[styleName]){ 
				return elem.style[styleName];  
			} else {
				 return elem.currentStyle[styleName];
			}
		}
	} else if(window.getComputedStyle) {
		getStyle = function(elem, styleName) {
			if(elem.style[styleName]){
				return elem.style[styleName];
			} else {
				return window.getComputedStyle(elem, null)[styleName];  
			}
		}
	} else {
		getStyle = function(elem, styleName) {
			return elem.style[styleName];
		}
	}
	return getStyle(elem, styleName);
} 
 

function getStyle(elem, styleName){
	if(elem.currentStyle) {
		getStyle = function(elem, styleName) {
			return elem.style[styleName] ? elem.style[styleName] : elem.currentStyle[styleName];
		}
	} else if(window.getComputedStyle) {
		getStyle = function(elem, styleName) {
			return elem.style[styleName] ? elem.style[styleName] : window.getComputedStyle(elem, null)[styleName];
		}
	} else {
		getStyle = function(elem, styleName) {
			return elem.style[styleName];
		}
	}
	return getStyle(elem, styleName);
}