<canvas id="canvas"></canvas>
<script type="text/JavaScript">
var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
canvas.width = window.innerWidth;
canvas.height = window.innerHeight;
// void ctx.rect(x, y, width, height);
ctx.rect(30, 30, 300, 250);
ctx.stroke();
</script>