'document.getElementById(...)' 为空或不是对象

<div id="form_div"></div>
<script type="text/javascript">
var urls = 'http://www.google.com/search?q=test';
var fragment = document.createDocumentFragment();
var form = document.createElement('form');
var input = document.createElement('input');
var submit = document.createElement('input');
form.action = urls;
form.setAttribute('method', 'post'); // 这里不能设置为get方法，因为GET方法会将action中原有的参数过滤后加个表单里的参数，如text1=value。
input.setAttribute('value', "value");
input.setAttribute('type', "text");
input.setAttribute('name', "text1");
input.setAttribute('size', "50");
submit.setAttribute('type', "submit");
form.appendChild(input); // node.appendChild(newNode)方法执行后返回的值是newNode。
form.appendChild(document.createElement('br'));
form.appendChild(submit);
fragment.appendChild(form);
try
{
 document.getElementById('form_div').appendChild(fragment);
}
catch (e)
{
 document.write(e.message);
 //document.getElementById("form_div") => null
 //FF: document.getElementById("form_div") has no properties
 //IE: 'document.getElementById(...)' 为空或不是对象 
 //document.write(e.description);
 //IE: 'document.getElementById(...)' 为空或不是对象 
}
</script>

必须将给以上代码写入body标签中才能正确运行。
不然这段代码在浏览器解析时生成的DOM中，这一段javascript被置于head中，而form_div这个Element则会出现在浏览器生成的DOM的body中，而页面js被执行时，因为这个Element在执行语句之后，所以就会找不到此Element，从而报对象为空或不是对象这个错误。

JavaScript Dom 修正

<script language="JavaScript" type="Text/JavaScript">
<!--
if(window.Event){// 修正Event的DOM
 /*
 IE5 MacIE5 Mozilla Konqueror2.2 Opera5
 event yes yes yes yes yes
 event.returnValue yes yes no no no
 event.cancelBubble yes yes no no no
 event.srcElement yes yes no no no
 event.fromElement yes yes no no no
 */
 Event.prototype.__defineSetter__("returnValue",function(b){//
  if(!b)this.preventDefault();
  return b;
 });
 Event.prototype.__defineSetter__("cancelBubble",function(b){// 设置或者检索当前事件句柄的层次冒泡
  if(b)this.stopPropagation();
  return b;
 });
 Event.prototype.__defineGetter__("srcElement",function(){
  var node=this.target;
  while(node.nodeType!=1)node=node.parentNode;
  return node;
 });
 Event.prototype.__defineGetter__("fromElement",function(){// 返回鼠标移出的源节点
  var node;
  if(this.type=="mouseover")
  node=this.relatedTarget;
  else if(this.type=="mouseout")
  node=this.target;
  if(!node)return;
  while(node.nodeType!=1)node=node.parentNode;
  return node;
 });
 Event.prototype.__defineGetter__("toElement",function(){// 返回鼠标移入的源节点
  var node;
  if(this.type=="mouseout")
  node=this.relatedTarget;
  else if(this.type=="mouseover")
  node=this.target;
  if(!node)return;
  while(node.nodeType!=1)node=node.parentNode;
  return node;
 });
 Event.prototype.__defineGetter__("offsetX",function(){
  return this.layerX;
 });
 Event.prototype.__defineGetter__("offsetY",function(){
  return this.layerY;
 });
}
if(window.Document){// 修正Document的DOM
 /*
 IE5 MacIE5 Mozilla Konqueror2.2 Opera5
 document.documentElement yes yes yes yes no
 document.activeElement yes null no no no

 */
}
if(window.Node){// 修正Node的DOM
 /*
 IE5 MacIE5 Mozilla Konqueror2.2 Opera5
 Node.contains yes yes no no yes
 Node.replaceNode yes no no no no
 Node.removeNode yes no no no no
 Node.children yes yes no no no
 Node.hasChildNodes yes yes yes yes no
 Node.childNodes yes yes yes yes no
 Node.swapNode yes no no no no
 Node.currentStyle yes yes no no no

 */
 Node.prototype.replaceNode=function(Node){// 替换指定节点
  this.parentNode.replaceChild(Node,this);
 }
 Node.prototype.removeNode=function(removeChildren){// 删除指定节点
  if(removeChildren)
  return this.parentNode.removeChild(this);
  else{
   var range=document.createRange();
   range.selectNodeContents(this);
   return this.parentNode.replaceChild(range.extractContents(),this);
  }
 }
 Node.prototype.swapNode=function(Node){// 交换节点
  var nextSibling=this.nextSibling;
  var parentNode=this.parentNode;
  node.parentNode.replaceChild(this,Node);
  parentNode.insertBefore(node,nextSibling);
 }
}
if(window.HTMLElement){
 HTMLElement.prototype.__defineGetter__("all",function(){
  var a=this.getElementsByTagName("*");
  var node=this;
  a.tags=function(sTagName){
   return node.getElementsByTagName(sTagName);
  }
  return a;
 });
 HTMLElement.prototype.__defineGetter__("parentElement",function(){
  if(this.parentNode==this.ownerDocument)return null;
  return this.parentNode;
 });
 HTMLElement.prototype.__defineGetter__("children",function(){
  var tmp=[];
  var j=0;
  var n;
  for(var i=0;i<this.childNodes.length;i++){
   n=this.childNodes[i];
   if(n.nodeType==1){
    tmp[j++]=n;
    if(n.name){
     if(!tmp[n.name])
     tmp[n.name]=[];
     tmp[n.name][tmp[n.name].length]=n;
    }
    if(n.id)
    tmp[n.id]=n;
   }
  }
  return tmp;
 });
 HTMLElement.prototype.__defineGetter__("currentStyle", function(){
  return this.ownerDocument.defaultView.getComputedStyle(this,null);
 });
 HTMLElement.prototype.__defineSetter__("outerHTML",function(sHTML){
  var r=this.ownerDocument.createRange();
  r.setStartBefore(this);
  var df=r.createContextualFragment(sHTML);
  this.parentNode.replaceChild(df,this);
  return sHTML;
 });
 HTMLElement.prototype.__defineGetter__("outerHTML",function(){
  var attr;
  var attrs=this.attributes;
  var str="<"+this.tagName;
  for(var i=0;i
   attr=attrs[i];
   if(attr.specified)
   str+=" "+attr.name+'="'+attr.value+'"';
  }
  if(!this.canHaveChildren)
  return str+">";
  return str+">"+this.innerHTML+"";
 });
 HTMLElement.prototype.__defineGetter__("canHaveChildren",function(){
  switch(this.tagName.toLowerCase()){
   case "area":
   case "base":
   case "basefont":
   case "col":
   case "frame":
   case "hr":
   case "img":
   case "br":
   case "input":
   case "isindex":
   case "link":
   case "meta":
   case "param":
   return false;
  }
  return true;
 });

 HTMLElement.prototype.__defineSetter__("innerText",function(sText){
  var parsedText=document.createTextNode(sText);
  this.innerHTML=parsedText;
  return parsedText;
 });
 HTMLElement.prototype.__defineGetter__("innerText",function(){
  var r=this.ownerDocument.createRange();
  r.selectNodeContents(this);
  return r.toString();
 });
 HTMLElement.prototype.__defineSetter__("outerText",function(sText){
  var parsedText=document.createTextNode(sText);
  this.outerHTML=parsedText;
  return parsedText;
 });
 HTMLElement.prototype.__defineGetter__("outerText",function(){
  var r=this.ownerDocument.createRange();
  r.selectNodeContents(this);
  return r.toString();
 });
 HTMLElement.prototype.attachEvent=function(sType,fHandler){
  var shortTypeName=sType.replace(/on/,"");
  fHandler._ieEmuEventHandler=function(e){
   window.event=e;
   return fHandler();
  }
  this.addEventListener(shortTypeName,fHandler._ieEmuEventHandler,false);
 }
 HTMLElement.prototype.detachEvent=function(sType,fHandler){
  var shortTypeName=sType.replace(/on/,"");
  if(typeof(fHandler._ieEmuEventHandler)=="function")
  this.removeEventListener(shortTypeName,fHandler._ieEmuEventHandler,false);
  else
  this.removeEventListener(shortTypeName,fHandler,true);
 }
 HTMLElement.prototype.contains=function(Node){// 是否包含某节点
  do if(Node==this)return true;
  while(Node=Node.parentNode);
  return false;
 }
 HTMLElement.prototype.insertAdjacentElement=function(where,parsedNode){
  switch(where){
   case "beforeBegin":
   this.parentNode.insertBefore(parsedNode,this);
   break;
   case "afterBegin":
   this.insertBefore(parsedNode,this.firstChild);
   break;
   case "beforeEnd":
   this.appendChild(parsedNode);
   break;
   case "afterEnd":
   if(this.nextSibling)
   this.parentNode.insertBefore(parsedNode,this.nextSibling);
   else
   this.parentNode.appendChild(parsedNode);
   break;
  }
 }
 HTMLElement.prototype.insertAdjacentHTML=function(where,htmlStr){
  var r=this.ownerDocument.createRange();
  r.setStartBefore(this);
  var parsedHTML=r.createContextualFragment(htmlStr);
  this.insertAdjacentElement(where,parsedHTML);
 }
 HTMLElement.prototype.insertAdjacentText=function(where,txtStr){
  var parsedText=document.createTextNode(txtStr);
  this.insertAdjacentElement(where,parsedText);
 }
 HTMLElement.prototype.attachEvent=function(sType,fHandler){
  var shortTypeName=sType.replace(/on/,"");
  fHandler._ieEmuEventHandler=function(e){
   window.event=e;
   return fHandler();
  }
  this.addEventListener(shortTypeName,fHandler._ieEmuEventHandler,false);
 }
 HTMLElement.prototype.detachEvent=function(sType,fHandler){
  var shortTypeName=sType.replace(/on/,"");
  if(typeof(fHandler._ieEmuEventHandler)=="function")
  this.removeEventListener(shortTypeName,fHandler._ieEmuEventHandler,false);
  else
  this.removeEventListener(shortTypeName,fHandler,true);
 }
}
//-->
</script>

此程序由某文章回帖处转过来的，未知原作者。