如何用画布轮廓突出显示图像地图区域？

// Canvas element
var canvas = document.getElementById('myCanvas');
// Drawing context
var hdc = canvas.getContext('2d');

// Event listeners
for (var i = 0; i < areas.length; i++) {
  areas[i].addEventListener('mouseover', function() {
    var coords = this.getAttribute('coords');
    hdc.beginPath();
    hdc.moveTo(coords[0], coords[1]);
    for (var j = 2; j < coords.length; j += 2) {
      hdc.lineTo(coords[j], coords[j+1]);
    }
    hdc.lineTo(coords[0], coords[1]);
    hdc.stroke();
  });
  areas[i].addEventListener('mouseout', function() {
    hdc.clearRect(0, 0, canvas.width, canvas.height);
  });
}