JavaScript checkbox help value PHP AJAX

I need help getting the value of a checkbox in a form I have. The HTML code for the checkbox is:

<INPUT TYPE="CHECKBOX" NAME="agreement" id="agreement" VALUE="iagree">

The JavaScript code is:

      var obj15 = document.getElementById("agreement");
      var agreement = obj15.value;

The PHP code to validate this is:

if ($agreement=="")
echo 'document.getElementById("emailsent").innerHTML = "You must agree to our Legal Notice and Privacy Statement before submission.";';

Now, the issue is that I cant get the JavaScript right to get the value of the checkbox. I need some help with this. Thanks. Also, if when you help me, please let me know what the value of false would be, is it an empty string?.... "" ? Thanks
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The value of the checkbox is always the value you define. VALUE="iagree"

Now to check if it is selected or not you need to check if it is checked. If it is checked it will return true , if it is not checked it will return false.

Here is a little example of how this would work

<script language="javascript">
       function check() {
        var obj15 = document.getElementById("agreement");
      var agreement = obj15.checked;
        if(agreement == true) {
              alert("Checkbox is checked");
            alert("Checkbox is NOT checked");

<INPUT TYPE="CHECKBOX" NAME="agreement" onChange="check()" id="agreement" VALUE="iagree">

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Have you used the same id multiple times --> (id="agreement" ). Verify, if that is the case try giving unique ids and check them
Another thing is your javascript code a part of the file that is called by Ajax. In that case you should move your javascript code to some other js file.
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