lineCap : shape of the end point of line

void gctx.lineCap='square';

var my_canvas=$('#my_canvas').get(0)
var gctx = my_canvas.getContext("2d");
gctx.beginPath()
gctx.moveTo(250,20);
gctx.lineTo(250,180);
gctx.lineCap='square';
gctx.lineWidth=20; 
gctx.stroke();