JSFiddle - React, Tailwind, and code Playground

HTML

<input type="number" id="times" value="400000"/>
<button id="go">simulate</button>
<p id="res"></p>
<p id="time"></p>

JavaScript

function monty(times) {
    var x, y, i,
        within = 0;

    for (i = 0; i < times; i = i + 1) {
        x = Math.random();
        y = Math.random();

        if (x * x + y * y < 1) {
            within = within + 1;
        }
    }

    return within * 4 / times;
}

function sim() {
    var before, time, res, times;

    times = document.getElementById("times").value;

    before = Date.now();
    res = monty(times);
    time = Date.now() - before;

    document.getElementById("res").innerHTML = "π ≈ " + res;
    document.getElementById("time").innerHTML = time + " ms";
}

document.getElementById("go").onclick = sim;