JSFiddle - React, Tailwind, and code Playground

HTML

<div class="form">
    <label for="units">Units:</label>
    <input type="text" name="units" id="units" class="required digits"  />
</div>

<div class="form">
    <label for="from">Convert From:</label>
    <select name="from" id="from">
        <option value="from">-Select an Option-</option>
        <option value="degC" selected="selected">Celcius</option>
    </select>
</div>

<div class="form">
    <label for="to">Convert Into:</label>
    <select name="to" id="to">
        <option value="to">-Select an Option-</option>
        <option value="degF" selected="selected">Fahrenheit</option>
    </select>
</div>

<div class="form">
    <input id="btnConvert" type="submit" name="submit" value="Convert!" />
</div>

<div class="form">
    <label> Result: </label>
    <input id="result" type="text" name="res" value="" />
</div>

JavaScript

function Convert(from, to, units, res){
    
    this.from = from;
    this.to = to;
    this.units = units;
    this.res = res;
}

Convert.prototype.convertThem = function(){

    var result = "invalid input";
    var input = parseInt(this.units.val());
    var f = this.from.val();
    var t = this.to.val()
    
    if(!isNaN(input)) {
        if(f == "degC"){
            if(t == "degF"){
                result =  input * 1.8 + 32;
            }
        }
    }            
    console.log('Parsed input is', input, "and result is", result);
    this.res.val(result);
    return result;
}

var calcTempTest = new Convert($("#from"), $("#to"), $("#units"), $("#result"));
$("#btnConvert").click(calcTempTest.convertThem.bind(calcTempTest));