<meta charset="utf-8"> 
<canvas width="600" height="400" ></canvas> 
<script>
    var tela = document.querySelector("canvas"); 
    var pincel = tela.getContext("2d"); 
    function triangulo(xa, ya, xc, yc, color){
        pincel.fillStyle = (color);
        pincel.beginPath(); 
        pincel.moveTo(xa, ya); 
        pincel.lineTo(xa, yc); 
        pincel.lineTo(xc, yc); 
        pincel.fill();  
    }
    triangulo(50, 50, 400, 50, "black");  
</script> 
            