js怎么获取css属性

window.onload = function() {
var but = document.getElementById('button');
var div = document.getElementById('getStyle');
but.onclick = function() {
alert(div.style.width);//弹出空的对话框
}
}

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

a = div.currentStyle;
alert(a.width);