I have 2 select lists in html, place to enter a number and second place to show the result. It works like a conversion, so when you choose milimeters from the first list and meters from the second it will show you number you entered x 0.001. There is no problem with small numbers (up to 9 digits - I can round these), but when I'm trying to do 1 millimeter to 1 yoctometer (1.0 × 10^-21 millimeters) sometimes it is correct, sometimes not. If I add zeros to yoctometer sometimes it will show something like 0.1 + 0.2 = 0.30000000000000004. The problem is I can't round it, because someone can enter like 1000 yoctometers and it won't work. Is there a way to fix this? (look at millimeters -> centimeters, Math.pow)

const config = {
    "milimeters": {
        "milimeters": v => v * 1,
        /* this is only an example */
        "centimeters": v => v * Math.pow(10, -21),
        "decimeters": v => v * 0.01,
        "meters": v => v * 0.001,
        "dekameters": v => v * 0.0001,
        "hectometers": v => v * 0.00001,
        "kilometers": v => v * 0.000001,
        "inches": v => v * 0.0393700787,
        "feet": v => v * 0.0032808399,
        "yards": v => v * 0.0010936133,
        "miles": v => v * 0.000000621371192, 
    },
}

function calculate() {
    const listFromV = document.getElementById("listFrom").value;
    const listToV = document.getElementById("listTo").value;
    const inputPlace = parseFloat(document.getElementById("inputPlace").value);

    const fn = config[listFromV][listToV];
    document.getElementById("resultPlace").innerHTML = fn(inputPlace);

    if (document.getElementById("inputPlace").value == "") {
        document.getElementById("resultPlace").innerHTML = "0.00";
    }

};

Edit:

1 * Math.pow(10, -21) = 1.0000000000000001e-21 /* not correct */
1000 * Math.pow(10, -21) = 1e-18 /* correct */
10 * Math.pow(10, -29) = 1.0000000000000001e-28 /* not correct */
100 * Math.pow(10, -29) = 1e-27 /* correct */