Draw line between two points in JavaScript?


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>

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>