Canvas mini painter

HTML

<canvas></canvas>

JavaScript

var canvas = document.querySelector('canvas');
var context = canvas.getContext('2d');

var x0 = canvas.width/2 - 15;
var y0 = canvas.height/2 -15;

canvas.addEventListener('mousemove', function(evt) { 
      //canvas.width = canvas.width;
      context.clearRect(0,0,canvas.width, canvas.height)
      context.fillRect(evt.x-15, evt.y-15,30,30);
    
}, false);