getElementById在任意一款浏览器中都可以用吗的疑问回复_基础知识

function $(objectId) {
if(document.getElementById && document.getElementById(objectId)) {
// W3C DOM
return document.getElementById(objectId);
}
else if (document.all && document.all(objectId)) {
// MSIE 4 DOM
return document.all(objectId);
}
else if (document.layers && document.layers[objectId]) {
// NN 4 DOM.. note: this won't find nested layers
return document.layers[objectId];
}
else {
return false;
}
}