Copy clipboard function supported by Firefox, Chrome and IE

function copyToClipboard(txt) {
 if (window.clipboardData) {
  window.clipboardData.clearData();
  window.clipboardData.setData("Text", txt);
  alert("复制成功！")
 } else if (navigator.userAgent.indexOf("Opera") != -1) {
  [removed] = txt;
  alert("复制成功！");
 } else if (window.netscape) {
  try {
   netscape.security.PrivilegeManager.enablePrivilege("UniversalXPConnect");
  } catch (e) {
   alert("被浏览器拒绝！\n请在浏览器地址栏输入'about:config'并回车\n然后将 'signed.applets.codebase_principal_support'设置为'true'");
  }
  var clip = Components.classes['@mozilla.org/widget/clipboard;1'].createInstance(Components.interfaces.nsIClipboard);
  if (!clip)
   return;
  var trans = Components.classes['@mozilla.org/widget/transferable;1'].createInstance(Components.interfaces.nsITransferable);
  if (!trans)
   return;
  trans.addDataFlavor('text/unicode');
  var str = new Object();
  var str = Components.classes["@mozilla.org/supports-string;1"].createInstance(Components.interfaces.nsISupportsString);
  var copytext = txt;
  str.data = copytext;
  trans.setTransferData("text/unicode", str, copytext.length * 2);
  var clipid = Components.interfaces.nsIClipboard;
  if (!clip)
   return false;
  clip.setData(trans, null, clipid.kGlobalClipboard);
  alert("复制成功！")
 }else if(copy){
  copy(txt);
  alert("复制成功！")
 }
}