javascript - 奇怪的 javascript 语法 return {unsubscribe() {}};

Question

我在 angular tutorial 中发现了奇怪的结构.返回部分发生了什么？代码块内的函数调用后跟空对象？

// This function runs when subscribe() is called
function sequenceSubscriber(observer) {
  // synchronously deliver 1, 2, and 3, then complete
  observer.next(1);
  observer.next(2);
  observer.next(3);
  observer.complete();

  // unsubscribe function doesn't need to do anything in this
  // because values are delivered synchronously
  return {unsubscribe() {}};
}

// Create a new Observable that will deliver the above sequence
const sequence = new Observable(sequenceSubscriber);

// execute the Observable and print the result of each notification
sequence.subscribe({
  next(num) { console.log(num); },
  complete() { console.log('Finished sequence'); }
});

// Logs:
// 1
// 2
// 3
// Finished sequence

Answer 1

您说代码 return {unsubscribe() {}}; 是“代码块内的函数调用，后跟空对象”；那是不正确的。

实际发生的是函数 sequenceSubscriber 返回一个 Object，其属性为名为“unsubscribe”的函数。该功能什么都不做。这是利用您可以在此处看到的新函数速记:Method definitions

考虑代码:

const foo = {
    bar(){}
};

创建一个对象foo，它有一个函数，bar，什么都不做。

为什么这样做的原因是为了实现 Rx 为 Observables 定义的接口(interface)契约以及定义在 Subscription 中的接口(interface)tc39/proposal-observable :

interface Subscription {

    // Cancels the subscription
    unsubscribe() : void;

    // A boolean value indicating whether the subscription is closed
    get closed() : Boolean;
}