根据其他输入更新表单输入字段值

Question

I am trying to build a calculator of sorts where you an input all but one variable and get the final variable autofilled in the form. I am trying this using a javascript function oninput for the form. However I am not sure how to achieve this. I have written a basic example of what I want below (though this does not work):

<form oninput="thermal()">
        <input type="number" name="avgTemp" id="avgTemp" class="five-col">
        <input type="number" name="volume" id="volume" class="five-col"> 
        <input type="number" name="deltaTemp" id="deltaTemp" class="five-col">
        <input type="number" name="Q" id="Q" class="five-col">
</form>

    function thermal(){
        var volume = document.getElementById("volume");
        var deltaTemp= document.getElementById("deltaTemp");
        value = deltaTemp;
    }

Answer 1

function thermal(){
    let avgTemp = document.getElementById("avgTemp").value;
    let volume = document.getElementById("volume").value;
    let deltaTemp = document.getElementById("deltaTemp").value;
    let q = document.getElementById("Q") // no value here yet; we will make this our total

    console.log(avgTemp, volume, deltaTemp, q)
 }

Well, let's start with this. This gives you each input value individually - from here, we can implement the pertinent logic to combine input values into one input field. I will make the Q input our "total" just so you can get a picture of how combining may look.

function thermal(){
    let avgTemp = document.getElementById("avgTemp").value;
    let volume = document.getElementById("volume").value;
    let deltaTemp = document.getElementById("deltaTemp").value;
    let q = document.getElementById("Q");

    let total = Number(avgTemp) + Number(volume) + Number(deltaTemp)
    console.log(total)
    q.value = total
    }

I am not sure what you are looking for but this should give you a decent head start. Fire away if you have any questions.

If you want to play around with it, here is the codepen: https://codepen.io/Pulse3358/pen/ExPZaVM