JavaScript - Set control's disabled not successful

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Javascript

In my project, I set a button disabled value dynamically. However, the button is always NOT grayed. When I debugged the script, the button's disabled value is always 'false', which means my code does work here (works well for some other controls). What's the possible bug source that causes the problem? Thanks.

function validate_Panel7() {
    debugger;
    var v = document.getElementById('btnValidate7');
    if (v.disabled == false)
        v = true;
    else
        return;
 }