My string variable doesn't replace the number in my if statement [duplicate]

I want to print the err variable to the payment Id if downtimeTotal is NaN or null or 0 (so essentially if the user doesn't put any inputs) but the calculator still prints NaN.

    <div class="totalwrapper">Credit Total:$<span id="payment" class="output"></span></div>
    </div>
    <button id="btn">Calculate</button>
    

    <script> 
    let btn = document.getElementById('btn')
    btn.addEventListener('click', function(){
        let err = "you didn't put any numbers in!"
        let rmr = document.getElementById('RMR').value;
        let cam = document.getElementById('bcam').value;
        let sens = document.getElementById('sensors').value;
        let dayMonth = document.getElementById('days').value;
        let bSensors = document.getElementById('badSensors').value;
        let down = document.getElementById('daysDown').value;
        let rmrCam = cam * 5 / dayMonth * down;
        let getDowntime = rmr / sens / dayMonth * bSensors * down + rmrCam;
        if (sens == 0 || NaN || null ) {
            getDowntime = rmr / dayMonth * bSensors * down + rmrCam;
        }
        let downtimeTotal = getDowntime.toFixed(2)
        if (downtimeTotal == NaN || null || 0.00){
            document.getElementById("payment").innerHTML = err;
        }else {
            document.getElementById("payment").innerHTML = downtimeTotal;
        }
    
    
    })
    </script>