converting negative farenheit number in javascript

Question

I can not get this code to convert a negative Fahrenheit value to a Celsius value using JavaScript. I'm assuming it is a problem with using either floor or isValid, but not sure. My code is successfully converting positive Fahrenheit values to Celsius, but not negative. this is also a program for converting fahrenheit to celsius, miles to kilometers and ounces to grams to clarify

   //alert("Input value validated as numeric: " + iv);
      var cT = document.getElementsByName("cType");
         if (cT[0].checked) {
             //to celsius
             if (isValid(iv,0)) {
             $("result").value = iv + " farenheit = " + FtoC(iv)  +    " celcius.";
             }
         }

  function FtoC (farenheit) {
      var C = (farenheit-32) * (5/9);
      return C;
  }
  function MtoK (miles) {
      var K = miles * 1.6;
      return K;
  }
  function OtoG (oz) {
      var g = oz * 28.35;
      return g;toFixed(3);
  }
  function isValid(val,floor) {
      if (val < floor) {
          return false;
      }
      return true;
   }

Answer 1

Here is a super-simple fahrenheit to celsius converter.

 const a = document.getElementById("answer"); const b = document.getElementById("butt"); const c = document.getElementById("convert"); var cels = 0; b.addEventListener("click", function() { var fahr = Number(c.value); cels = (1.8*fahr) + 32; a.innerHTML = cels + "<sup>o</sup>C"; }) 

 <input id="convert" type="number" /><sup>o</sup>F <button id="butt">Convert Me!</button> <p id="answer"></p>