Javascript issue - intermittently cannot find element using getElementById

I am having a problem with some javascript that I wrote that is only working intermittently. I dont know a huge amount about how things work in the DOM but I suspect that the JS that I have written is trying to access an element in the DOM before it exists. I wrote the JS to add it to a formassembly form. I do not have access to the actual code in the form to make changes there, I can only make JS changes.

Here is the link to the form http://www.tfaforms.com/186347

and this is the javascript that I am using on document load

 window.onload=function() {

document.getElementById("submit-").style.visibility = "hidden";
document.getElementById("wfPageNextId1").value="Calculate";
document.getElementById('wfPageNextId1').setAttribute('onclick', 'Calculate2011()'开发者_如何学JAVA)

}

I am getting errors saying that wfPageNextId1 is null and setAttribute cannot be called.

Any help would be very much appreciated. Thanks in advance.

---

It looks like some of the DOM is getting created dynamically by other scripts. You can delay until the elements exist via something along these lines:

function initializeForm() {
   var submit = document.getElementById("submit-"),
       nextPage = document.getElementById("wfPageNextId1");
   if (submit && nextPage) {
      submit.style.visibility = "hidden";
      nextPage.value = "Calculate";
      nextPage.setAttribute('onclick', 'Calculate2011()');
   } else {
      // try again in 1 second
      setTimeout(initializeForm, 1000);
   }
}

window.onload = function() {
   initializeForm();
};

You should also make sure you are not nuking any window.onload defined in the other scripts:

if (window.onload) {
   var currentOnload = window.onload;
   window.onload = function() { currentOnload(); initializeForm(); };
} else {
   window.onload = function() { initializeForm(); };
}

---

Checked the page HTML and infact there is no HTML element with the id wfPageNextId1. What is the element on the page that you are trying to access?