index.js

document.getElementById("submitButton").onclick = function () {
    let temp = Number(document.getElementById("textBox").value);

    const sourceUnitElement = document.querySelector('input[name="fromUnit"]:checked');
    const targetUnitElement = document.querySelector('input[name="toUnit"]:checked');

    if (!sourceUnitElement || !targetUnitElement) {
        displayMessage("You need to choose the temperature units");
        return;
    }

    const sourceUnit = sourceUnitElement.value;
    const targetUnit = targetUnitElement.value;
    const targetUnitSymbol = targetUnitElement.dataset.symbol;

    // we use kelvin as the base temp, this removes the need to account for possible combination of selections
    var converter = {
        celsius: {
            toKelvin: temp => temp + 273.15,
            fromKelvin: temp => temp - 273.15
        },
        fahrenheit: {
            toKelvin: temp => (temp - 32) * 5 / 9 + 273.15,
            fromKelvin: temp => (temp - 273.15) * 9 / 5 + 32
        },
        rankine: {
            toKelvin: temp => temp * 5 / 9,
            fromKelvin: temp => temp * 1.8
        },
        romer: {
            toKelvin: temp => (temp - 7.5) / 0.52500 + 273.15,
            fromKelvin: temp => (temp - 273.15) * 21 / 40 + 7.5
        },
        reaumur: {
            toKelvin: temp => temp * 4 / 5 + 273.15,
            fromKelvin: temp => (temp - 273.15) * 0.8
        },
        newton: {
            toKelvin: temp => temp * 100 / 33 + 273.15,
            fromKelvin: temp => (temp - 273.15) * 0.33000
        },
        delisle: {
            toKelvin: temp => 373.15 - (temp * 2 / 3),
            fromKelvin: temp => (temp - 273.15) * 1.5000 - 100.00
        },
    };

    if (sourceUnit != "kelvin") temp = converter[sourceUnit].toKelvin(temp);
    if (targetUnit != "kelvin") temp = converter[targetUnit].fromKelvin(temp);
    displayMessage(`${temp}°${targetUnitSymbol}`);
}

function displayMessage(string) {
    document.getElementById("result").innerHTML = string;
}