Call Javascript function only when checkbox is NOT checked

document.getElementById('icd').onchange = function() {
    if ( document.getElementById('icd').checked === false ) {
        planhide();
    }
};

Include onchange option in the input tag and then add an intermediate function that checks and calls planhide() accordingly as follows:

<input type="checkbox" id="icd" name="icd" value="icd" onchange=check()/>

Then define the check() to do check the state and call the function as follows:

function check()
{
if(document.getElementById("icd").checked==false)
planhide();
}

Also instead of onchange you can also use onclick on the submit button option to call the check() function as like follows:

<input type="button" onclick=check()/>

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