var canvas = document.createElement("canvas");
canvas.setAttribute("width", window.innerWidth);
canvas.setAttribute("height", window.innerHeight);
document.body.appendChild(canvas);
var ctx = canvas.getContext("2d");
ctx.fillStyle = "rgb(0, 0, 255)"; // blue
ctx.fillRect(100, 100, 500, 1);
/*
run:
draw a 500 pixels width blue line
*/