window.addEventListener("load", () => main());

const main = () => {
    const cnv = document.getElementById("canvas");
    const ctx = cnv.getContext("2d");

    const size = 100;

    ctx.fillStyle = "blue";
    ctx.fillRect(
        cnv.width / 2 - size / 2,
        cnv.height / 2 - size / 2,
        size,
        size
    );
};