qwe.ts

type NodeId = string;

type Node_ = {
  id: NodeId,
  label: string,
  collapsed: boolean,
  children: Node_[]
}

type Tree = Node_[]

function makeEmptyTree(): Tree {
  return [];
}

function traverseTree(tree: Tree, f: (node: Node_) => Node_): Tree {
  function go(node: Node_): Node_ {
    const children = node.children.map(go);
    return f({ ...node, children });
  }

  return tree.map(go);
}

function toggleNode(tree: Tree, nodeId: NodeId) {
  return traverseTree(tree, node => {
    if (node.id === nodeId) {
      return { ...node, collapsed: !node.collapsed };
    } else {
      return node;
    }
  })
}

function addToRoot(tree: Tree, node: Node_): Tree {
  return [...tree, node];
}

function addToNode(tree: Tree, newNode: Node_, targetId: NodeId): Tree {
  return traverseTree(tree, node => {
    if (node.id === targetId) {
      return { ...node, children: [...node.children, newNode] }
    } else {
      return node;
    }
  })
}

type Action =
  | { type: "toggle", nodeId: NodeId }
  | { type: "add_to_root", node: Node_ }
  | { type: "add_to_node", node: Node_, targetId: NodeId };

function reducer(state: Tree, action: Action): Tree {
  switch (action.type) {
    case "toggle": return toggleNode(state, action.nodeId);
    case "add_to_root": return addToRoot(state, action.node);
    case "add_to_node": return addToNode(state, action.node, action.targetId);
    default: return state;
  }
}