index.js


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

const canvasOffsetX = canvas.offsetLeft;
const canvasOffsetY = canvas.offsetTop;

canvas.width = window.innerWidth - canvasOffsetX * 2;
canvas.height = window.innerHeight - canvasOffsetY * 2;

let isPainting = false;
let lineWidth = 5;
let startX;
let startY;
ctx.strokeStyle = "skyblue";

const draw = (e) => {
    console.log(e);
    if (!isPainting) {
        return;
    }

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

    ctx.lineTo(e.pageX - canvasOffsetX, e.pageY - canvasOffsetY);
    ctx.stroke();

}
canvas.addEventListener('mousedown', (e) => {
    isPainting = true;
});

canvas.addEventListener('mouseup', (e) => {
    isPainting = false;
    ctx.stroke();
    ctx.beginPath();
});

canvas.addEventListener('mousemove', draw);