Получение позиции мышки в canvas

function writeMessage(canvas, message) { 
  var context = canvas.getContext('2d'); 
  context.clearRect(0, 0, canvas.width, canvas.height); 
  context.font = '18pt Calibri'; 
  context.fillStyle = 'black'; 
  context.fillText(message, 10, 25); 
} 
 
function getMousePos(canvas, evt) { 
  var rect = canvas.getBoundingClientRect(); 
  return { 
    x: evt.clientX - rect.left, 
    y: evt.clientY - rect.top 
  }; 
} 
var canvas = document.getElementById('myCanvas'); 
var context = canvas.getContext('2d'); 
 
canvas.addEventListener('mousemove', function(evt) { 
  var mousePos = getMousePos(canvas, evt); 
  var message = 'Mouse position: ' + mousePos.x + ',' + mousePos.y; 
  writeMessage(canvas, message); 
}, false);

body { 
  margin: 0px; 
  padding: 0px; 
}

<canvas id="myCanvas" width="578" height="200"></canvas>