Implementing Canvas Functionality using JavaScript

Now that your Flask app displays a blank drawing canvas, let’s make it interactive! In this lesson, you’ll use JavaScript to turn your canvas into a real sketchpad where users can draw, erase, and submit their creations.

You will discover how to:

• Capture mouse events so users can draw lines on the canvas.
• Add buttons to clear the canvas or submit their drawings.
• Convert your drawing into a format that can be sent to the server for recognition.

To start, you’ll need to grab the canvas element and get its drawing context:

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

You’ll listen for mouse events to know when the user starts and stops drawing:

canvas.addEventListener('mousedown', (e) => {
    drawing = true;
    console.log('Started drawing');
    ctx.beginPath();
    ctx.moveTo(e.offsetX, e.offsetY);
});

canvas.addEventListener('mousemove', (e) => {
    if (drawing) {
        ctx.lineTo(e.offsetX, e.offsetY);
        ctx.stroke();
    }
});

canvas.addEventListener('mouseup', () => {
    drawing = false;
    console.log('Stopped drawing');
});

You’ll also set up the drawing style, like line width and shape:

ctx.lineWidth = 5;
ctx.lineCap = 'round';

You’ll add a Clear button so users can erase their drawing:

function clearCanvas() {
    ctx.clearRect(0, 0, canvas.width, canvas.height);
}

And a Submit button to send the drawing to your Flask backend:

function submitCanvas() {
    const dataURL = canvas.toDataURL();
    // Send dataURL to the server
}

Making your canvas interactive is a key step in building any drawing or recognition app. By learning how to handle user input and work with the HTML canvas, you gain skills that are useful in many web projects — from games to art tools to educational apps. Plus, you’ll see how JavaScript and Python can work together to create smart, interactive experiences.