javascript - 是否可以在没有异步/等待的情况下从 promise 返回已解析的值？

Question

我正在关注 apollo 教程 ( https://www.apollographql.com/docs/tutorial/resolvers/ )，我看到了这段代码:

me: async (_, __, { dataSources }) =>
  dataSources.userAPI.findOrCreateUser()

因为 dataSources.userAPI.findOrCreateUser() 返回 Promise，我想 await dataSources.userAPI.findOrCreateUser() 是正确的。

但它工作得非常好，没有任何错误，我在 React 中得到了解决的值(value)......即使下面的代码也工作得很好。

me: (_, __, { dataSources }) =>
  dataSources.userAPI.findOrCreateUser()

这段代码让我很困惑。它是如何工作的？

Answer 1

除了启用 await , async implicitly wraps函数的结果变成了 Promise.resolve() .大致:

async function() {
  return something;
}

相当于:

function() {
  return Promise.resolve(something);
}

事情是Promise.resolve() “扁平化”它的参数，这意味着如果它的参数是一个thenable(例如另一个 Promise)，它会自动“解决”它。换句话说，Promise.resolve(somethingThatIsAPromise).then(<work>)与somethingThatIsAPromise.then(<work>)效果相同.

MDN tries to explain that behavior (粗体是我的):

The Promise.resolve() method returns a Promise object that is resolved with a given value. If the value is a promise, that promise is returned; if the value is a thenable (i.e. has a "then" method), the returned promise will "follow" that thenable, adopting its eventual state; otherwise the returned promise will be fulfilled with the value. This function flattens nested layers of promise-like objects (e.g. a promise that resolves to a promise that resolves to something) into a single layer.

并且，由于箭头函数返回的 ( dataSources.userAPI.findOrCreateUser() ) 是一个 Promise，由于“扁平化”，具有 async或不会以相同的行为结束。