SOURCE

var c = document.getElementById("myCanvas");
c.width = 300;
c.height = 300;
var flagColor = 'blue',lineWeight=100,arcc='red', radi = c.width /2;

var cxt = c.getContext("2d"), w= c.width - lineWeight;

cxt.beginPath();
cxt.strokeStyle=arcc;
cxt.fillStyle="pink";
cxt.lineWeight=lineWeight
cxt.arc(c.width/2,c.height/2,c.height/2,0,2*Math.PI);
cxt.fill();
cxt.stroke();

cxt.beginPath();
cxt.moveTo(0, radi-lineWeight);
cxt.lineTo(c.width, c.height/2);
cxt.lineWeight = lineWeight;
cxt.strokeStyle = flagColor;
cxt.stroke(); 

cxt.beginPath();  //注释二
cxt.moveTo(c.width / 2, 0);
cxt.lineTo(c.width / 2, c.height);
cxt.lineWeight = lineWeight;
cxt.strokeStyle = flagColor;
cxt.stroke();

<canvas id="myCanvas" width="300" height="150" style="border:1px solid blue;">
Your browser does not support the HTML5 canvas tag.</canvas>