draw-geometry.js

// Project 3D points to 2D
const vertices2D = vertices.map(p => project(p));

// Draw each cell
for (let cell of cells) {
  // Get the 2D points that define this triangle or quad
	const points2D = cell.map(idx => vertices2D[idx]);
  
  // Draw this polygon
  context.beginPath();
  for (let [x, y] of points2D) {
    context.lineTo(x, y);
	}
  context.closePath();
  context.stroke();
}