var im = document.getElementById('picture')?

Question

var im = document.getElementById('picture');

function bigger(){

document.getElementById('picture').style.width = "400px";

im.style.width="400px"

document.getElementById('picture').style.height = "400px";

}

我想问为什么im.style.width="400px"  不能改变style  变量接收就不能改变style吗？