<body>
    <canvas id="canvas" width="200" height="200"></canvas>
    <script>
        const canvas = document.getElementById("canvas");
        const ctx = canvas.getContext("2d");
        const width = canvas.width;
        const height = canvas.height;

        ctx.fillStyle = "green";
        ctx.fillRect(0, 0, width / 2, height / 2);
    </script>
</body>