<body>
    <canvas id="target" width="400" height="400"></canvas>
    <script>
        const targetCanvas = document.getElementById('target');
        const target = targetCanvas.getContext('2d');

        const canvasWidth = targetCanvas.width;
        const canvasHeight = targetCanvas.height

        target.fillStyle = 'green';
        target.fillRect(0, 0, canvasWidth, canvasHeight);
    </script>
</body>