JS DOM: Stopwatch

Here is how to write a stopwatch.

Code

<div>

<p>Elapsed time:
<span id="output_rbFQx">0.0</span></p>

<button id="start_Qdnm6" type="button">Start</button>
<button id="stop_ZSxPC" type="button">Stop/Lap</button>
<button id="reset_fWZFm" type="button">Reset</button>

<script async src="js_stopwatch.js"></script>

</div>

{

const start_Qdnm6 = document.getElementById("start_Qdnm6");
const stop_ZSxPC = document.getElementById("stop_ZSxPC");
const reset_fWZFm = document.getElementById("reset_fWZFm");
const output_rbFQx = document.getElementById("output_rbFQx");

let StartedDateTime = new Date;
let ElapsedTime = 0;
let IntvID = null;
let TimerIsOn = false;
let TimerUpdateFrequency = 100; // millisecond

const fstart = (() => {
    StartedDateTime = new Date;
    fdisplay("start");
})

const fstop = (() => {
    ElapsedTime = Date.now () - StartedDateTime.getTime();
    fdisplay("stop");
});

const freset = (() => {
    ElapsedTime = 0;
    fdisplay("stop");
    output_rbFQx.firstChild.nodeValue = "0";
});

const updateDisplay = (() => {
    output_rbFQx.firstChild.nodeValue = ( (Date.now () - StartedDateTime.getTime()) / 1000 ).toFixed (2);
});

const fdisplay = ((p_startStop) => {
    if (p_startStop === "start") {
        if ( TimerIsOn === true ) {}
        else {
            TimerIsOn = true;
            IntvID = setInterval( updateDisplay , TimerUpdateFrequency);
        }
    } else {
        if (IntvID !== null ) {
            window.clearInterval(IntvID);
        }
        TimerIsOn = false;
        updateDisplay();
    }
});

start_Qdnm6.addEventListener("click", fstart, false);
stop_ZSxPC.addEventListener("click", fstop, false);
reset_fWZFm.addEventListener("click", freset, false);

}