SOURCE

 window.onload=eventWindowLoaded();

        
        function eventWindowLoaded() {
            canvasApp();
        }

        function canvasApp() {


            var canvas = document.getElementById('canvasOne');
            var ctx = canvas.getContext('2d');

            var w = canvas.width = 1020;
            var h = canvas.height = 1080;

            function reOffset() {
                var BB = canvas.getBoundingClientRect();
                offsetX = BB.left;
                offsetY = BB.top;
            }

            var offsetX, offsetY;
            reOffset();

            window.onscroll = function(e) {
                reOffset();
            }

            window.onresize = function(e) {
                reOffset();
            }

            canvas.addEventListener('mousemove', mouseMove, false);
            canvas.addEventListener('touchmove', mouseMove, false);

            function draw(cx, cy, radius) {
                ctx.save();
                ctx.clearRect(0, 0, w, h);

                var radialGradient = ctx.createRadialGradient(cx, cy, 1, cx, cy, radius);

                radialGradient.addColorStop(0, 'rgba(0, 0, 0, 1)');
                radialGradient.addColorStop(.65, 'rgba(0, 0, 0, 1)');
                radialGradient.addColorStop(1, 'rgba(0, 0, 0, 0)');

                ctx.beginPath();

                ctx.fillStyle = '#000';
                ctx.fillRect(0, 0, w, h);

                ctx.globalCompositeOperation = 'destination-out';

                ctx.arc(cx, cy, radius, 0, Math.PI * 2, false);
                ctx.fillStyle = radialGradient;
                ctx.fill();

                ctx.restore();
            }

            function mouseMove(e) {
                e.preventDefault();
                e.stopPropagation();

                mouseX = parseInt(e.clientX - offsetX);
                mouseY = parseInt(e.clientY - offsetY);

                draw(mouseX, mouseY, 100);
            }
            draw(w / 2, h / 2, 100);
        }

<canvas id="canvasOne" width="800" height="800"></canvas>

* {
            margin: 0;
            padding: 0;
            box-sizing: border-box;
        }
        
        html {
            width: 100vw;
            height: 100vh;
        }
        
        body {
            background: url("https://farm3.staticflickr.com/2908/32764885503_1a04915b11_k.jpg") no-repeat center;
            width: 100vw;
            min-height: 100%;
        }