<canvas id="canvas"></canvas>

const canvas = document.getElementById('canvas');
const ctx = canvas.getContext('2d');

canvas.width = window.innerWidth;
canvas.height = window.innerHeight;

canvasW = canvas.width;
canvasH = canvas.height;


ctx.beginPath();
ctx.moveTo(0,canvasH / 2);
ctx.lineTo(canvasW,canvasH / 2);
ctx.strokeStyle = "red";
ctx.lineWidth = 10;
ctx.stroke();
ctx.closePath();