<canvas width="600" height="400" ></canvas>
<script>
var tela = document.querySelector('canvas');
var pincel = tela.getContext('2d');
pincel.fillStyle = 'grey';
pincel.fillRect(0, 0, 600, 400);
function exibeAlerta(evento) {
alert('Cliquei');
}
tela.onclick = exibeAlerta;
</script>
