populate-tree.js

// tree is what is returned post shenanigans.
let tree:TreeItem[] = [];

// keyMap keeps track of the indexes where parentKeys are found in the keyed but flat array.
const keyMap:Map<string, number> = new Map();

treeItemsWithBranches.forEach( ( node ):void => {

  // nodes with no parents are at root level.
  // this is not a binary tree, there can be several root level nodes.       
  if ( !node.parentKey ) {
    tree.push( node );
    tree = tree.sort( sortByName );
    return;
  }

  // this could be left undefined, but Typescript preferred it this way 🧐 
  let parentIndex = keyMap.get( node.parentKey );

  if ( !keyMap.get( node.parentKey ) ) {

    parentIndex = treeItemsWithBranches.findIndex( ( el ):boolean => el.key === node.parentKey );

    keyMap.set( node.parentKey, parentIndex );
  }


  // from here it is just about putting balls in holes.
  if ( parentIndex !== undefined && parentIndex >= 0 ) { 
    if ( !treeItemsWithBranches[parentIndex] || !treeItemsWithBranches[parentIndex].children ) {
      treeItemsWithBranches[parentIndex].children = [node];
      return;
    }

    // object conditional chaining also brings me joy!
    treeItemsWithBranches[parentIndex]?.children?.push( node ); 
    
    // i'm keen to improve this as repeatedly sorting feels the most expensive part of this whole exercise.
    treeItemsWithBranches[parentIndex]?.children?.sort( sortByName );
  }
} );

return tree;