draw2.js

document.addEventListener('DOMContentLoaded', () => {

    // state
    let draw = false;

    // elements
    let points = [];
    let lines = [];
    let svg = null;

    function render() {

        // create the selection area
        svg = d3.select('#draw')
                .attr('height', window.innerHeight)
                .attr('width', window.innerWidth);

        svg.on('mousedown', function() {
            draw = true;
            const coords = d3.mouse(this);
            draw_point(coords[0], coords[1], false);
        });

        svg.on('mouseup', () =>{
            draw = false;
        });

        svg.on('mousemove', function() {
            if (!draw)
                return;
            const coords = d3.mouse(this);
            draw_point(coords[0], coords[1], true);
        });

        document.querySelector('#erase').onclick = () => {
            for (let i = 0; i < points.length; i++)
                points[i].remove();
            for (let i = 0; i < lines.length; i++)
                lines[i].remove();
            points = [];
            lines = [];
        }

    }

    function draw_point(x, y, connect) {

        const color = document.querySelector('#color-picker').value;
        const thickness = document.querySelector('#thickness-picker').value;

        if (connect) {
            const last_point = points[points.length - 1];
            const line = svg.append('line')
                            .attr('x1', last_point.attr('cx'))
                            .attr('y1', last_point.attr('cy'))
                            .attr('x2', x)
                            .attr('y2', y)
                            .attr('stroke-width', thickness * 2)
                            .style('stroke', color);
            lines.push(line);
        }

        const point = svg.append('circle')
                         .attr('cx', x)
                         .attr('cy', y)
                         .attr('r', thickness)
                         .style('fill', color);
        points.push(point);
    }

    render();
});