JSFiddle - React, Tailwind, and code Playground

HTML

<input id="fahrenheit"  placeholder="fahrenheit" onkeyup="tempConverter(event)"> = 
<input id="celsius"  placeholder="celsius" onkeyup="tempConverter(event)">

JavaScript

function tempConverter(event){
// using event object to get the parameters
var getTarget = event.target.id;
var getTargetValue = event.target.value;
  
  //When no input value is present  
    if(getTargetValue ==""){
      document.getElementById("celsius").value = "";
      document.getElementById("fahrenheit").value = "";
    }
    
   // populate the relevant input box 
    if(isNumber(getTargetValue)){
    
       if(getTarget ==="fahrenheit"){
           document.getElementById("celsius").value = getCelcius(getTargetValue);
       }
       else if(getTarget ==="celsius"){
            document.getElementById("fahrenheit").value = getFarenheit(getTargetValue);
       }
    }

}

// Convert from c->f
function getFarenheit(c){
  var f =0;
  f = 1.8*c+32;
  return f;

}

//Convert from f->c
function getCelcius(f){
   var c = 0;
   c = (f-32)/1.8;
   return c;
}

//test if it is number
function isNumber(test){
    return !isNaN(parseFloat(test)) && isFinite(test); 
 }