retrace.ts

  // did we find the destination?
  if (lowestCostNode.x === state.destX && lowestCostNode.y === state.destY) {
    // retrace our steps back to the beginning!
    function retrace(node, path) {
      // did we find the origin? if so, we're done!
      if (node.previous === undefined) return [node, ...path]
      // not yet ... let's keep retracing our steps
      return retrace(node.previous, [ node, ...path ])
    }
    // kick off the retracing!
    return retrace(lowestCostNode, [])
  }