javascript - 为什么分配给变量(别名条件)时对类属性的类型缩小检查不起作用？

Question

如何缩小别名条件表达式中类的属性类型？
短 :在一个类方法中，我想做一个类型缩小检查，例如 this._end === null && this._head === null ，但我想先将此检查的结果分配给一个变量，然后再在类型缩小 if 中使用它条款。它没有按预期工作。
不短 :假设我们有一个 Queue 类:

class Queue {
  private _length: number = 0;
  private _head: Node | null = null;
  private _end: Node | null = null;

  public enqueue(node: Node): boolean {
    this._length += 1;

    const isQueueEmpty = this._end === null;
    if (isQueueEmpty) {
      this._head = node;
      this._end = node;
      return true;
    }

    this._end.setLink(node); // TypeScript doesn't understand that this._end passed the null check 
                             // in the if statement and writes that this._end can be null. 
    this._end = node;
    return true;
  }
}

如果你直接在 中写入 enqueue 函数中的检查如果 条件，那么 TypeScript 就知道 this._end 不是 空 :

public enqueue(node: Node): boolean {
  this._length += 1;

  if (this._end === null) {
    this._head = node;
    this._end = node;
    return true;
  }

  this._end.setLink(node); // Now TypeScript understand that this._end passed the null check
  this._end = node;
  return true;
}

但是代码的描述性较差。
此外，总的来说，我需要说类的两个属性都通过了 。空 查看。具体来说:this._head 和 this._end。
我写了以下方法:

type QueuePointerKeys = '_head' | '_end';

private _isQueueEmpty(): this is this & { [K in QueuePointerKeys]: null } {
  return this._end === null;
}

它不起作用。如果你把它放在 enqueue 方法中，那么:

public enqueue(node: Node): boolean {
  this._length += 1;

  if (this._isQueueEmpty()) {
    this._head = node;     // TypeScript writes that a variable of type null cannot be assigned the type Node
    this._end = node;      // TypeScript writes that a variable of type null cannot be assigned the type Node
    return true;
  }

  this._end.setLink(node); // TypeScript doesn't understand that this._end passed the null check
                           // in the if statement and writes that this._end can be null.
  this._end = node;
  return true;
}

如何使用类方法，例如 _isQueueEmpty类型缩小？这与 this._end === null && this._head === null 类似。直接用作 的条件如果 陈述？

Answer 1

Control Flow Analysis of Aliased Conditions and Discriminants只是最近才添加的，它有一些限制，包括:

Narrowing through indirect references occurs only when the conditional expression or discriminant property access is declared in a const variable declaration with no type annotation, and the reference being narrowed is a const variable, a readonly property, or a parameter for which there are no assignments in the function body.

由于 this._end不是只读属性，它不会为你工作。我通过分配 this._end 确认这是您的问题到本地 const 变量，然后将其用于条件以及 setLink称呼:

interface Node {
  setLink(node: Node): void
}

class Queue {
  private _length: number = 0;
  private _head: Node | null = null;
  private _end: Node | null = null;

  public enqueue(node: Node): boolean {
    this._length += 1;

    const end = this._end
    const isQueueEmpty = end === null;

    if (isQueueEmpty) {
      this._head = node;
      this._end = node;
      return true;
    }

    end.setLink(node);
    this._end = node;
    return true;
  }

}

事实上，有一个针对您的具体案例的未解决问题: Control flow analysis of aliased conditions is not able to narrow object properties #46412 ，以及 Allow non-readonly properties to be used in aliased conditional expressions #44972 .
让我们希望他们能尽快找到解决方案🤞🏾