var div1 = document.getElementById("div1");
var oldwidth = div1.style.width;
oldwidth = parseInt(oldwidth, 10) + 50 + "px";
div1.style.width = oldwidth;
网页内容:
<div id="div1" style="width:200px;height:200px;border:1px solid #000;"></div>
<input type="button" value="大小增加50" οnclick="increase()" />