How to set the value of a input hidden field through JavaScript?

For me it works:

document.getElementById("checkyear").value = "1";
alert(document.getElementById("checkyear").value);

http://jsfiddle.net/zKNqg/

Maybe your JS is not executed and you need to add a function() {} around it all.

You need to run your script after the element exists. Move the <input type="hidden" name="checkyear" id="checkyear" value=""> to the beginning.

It seems to work fine in Google Chrome. Which browser are you using? Here the proof http://jsfiddle.net/CN8XL/

Anyhow you can also access to the input value parameter through the document.FormName.checkyear.value. You have to wrap in the input in a <form> tag like with the proper name attribute, like shown below:

<form name="FormName">
    <input type="hidden" name="checkyear" id="checkyear" value="">
</form>

Have you considered using the jQuery Library? Here are the docs for .val() function.

The first thing I will try - determine if your code with alerts is actually rendered. I see some server "if" code in what you posted, so may be condition to render javascript is not satisfied. So, on the page you working on, right-click -> view source. Try to find the js code there. Please tell us if you found the code on the page.