JSFiddle - React, Tailwind, and code Playground

HTML

<body onload="init()">
    <canvas id="under" width="400" height="400" style="position:absolute;top:10%;left:10%;border:2px solid;" z-index="1;"></canvas>
    <canvas id="can" width="400" height="400" style="position:absolute;top:10%;left:10%;border:2px solid;" z-index="2;"></canvas>
</body>

JavaScript

//inspired by http://stackoverflow.com/questions/2368784/draw-on-html5-canvas-using-a-mouse
var canvas, canvasUnder, ctx, ctxUnder, circle, drawing = false,
    prevX = 0,
    currX = 0,
    prevY = 0,
    currY = 0;

var x = "black",
    y = 2;

var w,h;

function init() {
    canvas = document.getElementById('can');
    ctx = canvas.getContext("2d");

    canvasUnder = document.getElementById('under');
    ctxUnder = canvasUnder.getContext("2d");

    w = canvas.width;
    h = canvas.height;
    canvas.addEventListener("mousemove", function (e) {
        findxy('move', e)
    }, false);
    canvas.addEventListener("mousedown", function (e) {
        findxy('down', e)
    }, false);
    canvas.addEventListener("mouseup", function (e) {
        findxy('up', e)
    }, false);
    canvas.addEventListener("mouseout", function (e) {
        findxy('out', e)
    }, false);

    ctx.rect(0,0,canvas.width,canvas.height);
	ctx.fillStyle="blue";
	ctx.fill();

    //fill the canvas under with solid color to start with
    ctxUnder.rect(0,0,canvasUnder.width,canvasUnder.height);
	ctxUnder.fillStyle="red";
	ctxUnder.fill();
}

function draw() {
    circle = new Path2D();
    circle.arc(currX,currY,16,0,2*Math.PI);
	ctx.clip(circle);
	ctx.clearRect(0, 0, w, h);
	ctx.restore();
}

function findxy(res, e) {
    if (res == 'down') {
        prevX = currX;
        prevY = currY;
        currX = e.clientX - canvas.offsetLeft;
        currY = e.clientY - canvas.offsetTop;

        drawing = true;
    }
    if (res == 'up' || res == "out") {
        drawing = false;
    }
    if (res == 'move') {
        if (drawing) {
            prevX = currX;
            prevY = currY;
            currX = e.clientX - canvas.offsetLeft;
            currY = e.clientY - canvas.offsetTop;
            draw();
        }
    }
}