Draw line between two points in JavaScript?

Question

I need to draw a line between where the mouse used to be and where the mouse is now. In my search for an answer I found https://www.w3schools.com/graphics/svg_line.asp which explains making a single static line in HTML but nothing to do with dynamically creating them in JS.

Here is the code I am working with so far. I can draw circles with every mouse movement but not draw a line between those two mouse movements.

 <script src="https://d3js.org/d3.v4.min.js"> </script> <svg id="svg" style="width:100%; height:800px" /> <script> const svg = d3.select('#svg'); let drawing = false; let previous_coords = null; function draw_point(previous_coords) { if (!drawing) return; const coords = d3.mouse(this); svg.append('circle') .attr('cx', coords[0]) .attr('cy', coords[1]) .attr('r', 5) .style('fill', 'black'); // this block doesn't work svg.append('line') .attr('x1', previous_coords[0]) .attr('y1', previous_coords[1]) .attr('x2', coords[0]) .attr('y2', coords[1]) .style('stroke', rgb(255, 0, 0)) .style('stroke-width', 10); previous_coords = d3.mouse(this); }; svg.on('mousedown', () => { drawing = true; }); svg.on('mouseup', () => { drawing = false; }); svg.on('mousemove', draw_point); </script>

Answer 1

I'm not sure d3 is the best possible choice of library for what you are trying to do, but your immediate issue is the variable previous_coords . You have declared it both as a global, and as an argument to your draw_point function. Since draw_point is called as an event handler for the mousemove event I don't think it will ever be passed a point as an argument. If you eliminate the extraneous argument declaration, then you still have the problem of not initializing previous_coords before you use it, but this can be solved by initializing previous_coords in the mouse down event. Here is an updated snippet that hopefully works as you intended:

 <script src="https://d3js.org/d3.v4.min.js"> </script> <svg id="svg" style="width:100%; height:800px" /> <script> const svg = d3.select('#svg'); let drawing = false; let previous_coords = null; function draw_point() { if (!drawing) return; const coords = d3.mouse(this); svg.append('circle') .attr('cx', previous_coords[0]) .attr('cy', previous_coords[1]) .attr('r', 5) .style('fill', 'black'); svg.append('circle') .attr('cx', coords[0]) .attr('cy', coords[1]) .attr('r', 5) .style('fill', 'black'); // this block doesn't work svg.append('line') .attr('x1', previous_coords[0]) .attr('y1', previous_coords[1]) .attr('x2', coords[0]) .attr('y2', coords[1]) .style('stroke', 'rgb(255, 0, 0)') .style('stroke-width', 2); previous_coords = coords; }; svg.on('mousedown', function() { previous_coords = d3.mouse(this) drawing = true; }); svg.on('mouseup', () => { drawing = false; }); svg.on('mousemove', draw_point); </script>