“节点”类型上不存在通用属性“启用”？

Question

There is a type Node :

export type Node<T> = T extends ITreeNode ? T : never;

export interface ITreeNode extends TreeNodeBase<ITreeNode> {
   enabled: boolean;
}
  
export abstract class Tree<Node> {
  nodeEnableToggle(node: Node): void {
    node.enabled = !node.enabled;
  }
}

Why I get this error message:

Property 'enabled' does not exist on type 'Node'.

if T extends ITreeNode ?

I also tried this:

type Node<T> = T & ITreeNode;

 initialize(source: Node): void {
    this.dataSource.data = source.children;
  }

The same error:

Property 'children' does not exist on type 'Node'

Answer 1

I have tried to recreate a reproducible example here. As far as I understand, if you absolutely have to use conditional type, you may require to pass two parameters to the generic. Both being the implementation of ITreeNode . There may be a better way to do this without repetition.

However, I was wondering what extra goals are you achieving by having conditional type that you cannot do by using non fancy extend as I have done in Tree1 class below?

TS Playground Link

interface TreeNodeBase<T> {
  obj: T;
}

export interface ITreeNode extends TreeNodeBase<ITreeNode> {
  enabled: boolean;
}
export type Node<T> = T extends ITreeNode ? T : never;

interface TreeActions<T> {
  action: (t: T) => void;
}

// removed abstract to show examples below
export  class Tree<U, T extends Node<U>> implements TreeActions<Node<U>> {
  nodeEnableToggle(node: T): void {
    node.enabled = !node.enabled;
  }

  action() {}
}

// removed abstract to show examples below
export  class Tree1<T extends ITreeNode> implements TreeActions<T> {
  nodeEnableToggle(node: T): void {
    node.enabled = !node.enabled;
  }

  action() {}
}

// Examples
interface ATreeNode extends ITreeNode {
    extra: number;
}
new Tree<ATreeNode, ATreeNode>();
new Tree1<ATreeNode>();