<canvas id="canvas" width="500" height="500" style="background-color: yellow;"></canvas>

var canvas=document.getElementById("canvas");

var cxt=canvas.getContext("2d");

cxt.beginPath();

cxt.moveTo(250,50);

cxt.lineTo(200,200);

cxt.lineTo(300,300);

cxt.closePath();//填充或闭合 需要先闭合路径才能画

//空心三角形

cxt.strokeStyle="red";

cxt.stroke();

//实心三角形

cxt.beginPath();

cxt.moveTo(350,50);

cxt.lineTo(300,200);

cxt.lineTo(400,300);

cxt.closePath();

cxt.fill();