js怎么获取css属性

window.onload = function() {

var but = document.getElementById('button');

var div = document.getElementById('getStyle');

but.onclick = function() {

var a = document.defaultView.getComputedStyle(div);

alert(a.width);//100px

}

}

alert(a.background);//reb(255,0,0) none repeat sroll 0% 0% / auto padding-box border-box

alert(a.backgroundColor);//red