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假设在

then()

then()

let p2 = p1.then(() => {  throw new Error('lol')})// p2 was rejected with Error('lol')let p3 = p1.then(() => {  return 42})// p3 was fulfilled with 42

但是有时候，即使在延续中，我们也不知道我们是否成功。我们需要更多时间。

return checkCache().then(cachedValue => {  if (cachedValue) {    return cachedValue  }  // I want to do some async work here})

但是，如果我在那里进行异步工作，那么对

return

throw

return checkCache().then(cachedValue => {  if (cachedValue) {    return cachedValue  }  fetchData().then(fetchedValue => {    // Doesn’t make sense: it’s too late to return from outer function by now.    // What do we do?    // return fetchedValue  })})

这就是为什么如果您 无法解决另一个Promise的原因，Promise 不会有用。

这并不意味着在您的示例

p2

p3

p2

then()

p3

p3

p2

至于这种情况 如何
发生，则取决于具体实现。在内部，您可以将其

then()

// Warning: this is just an illustration// and not a real implementation pre.// For example, it completely ignores// the second then() argument for clarity,// and completely ignores the Promises/A+// requirement that continuations are// run asynchronously.then(callback) {  // Save these so we can manipulate  // the returned Promise when we are ready  let resolve, reject  // Imagine this._onFulfilled is an internal  // queue of pre to run after current Promise resolves.  this._onFulfilled.push(() => {    let result, error, succeeded    try {      // Call your callback!      result = callback(this._result)      succeeded = true    } catch (err) {      error = err      succeeded = false    }    if (succeeded) {      // If your callback returned a value,      // fulfill the returned Promise to it      resolve(result)    } else {      // If your callback threw an error,      // reject the returned Promise with it      reject(error)    }  })  // then() returns a Promise  return new Promise((_resolve, _reject) => {    resolve = _resolve    reject = _reject  })}

同样，这是非常伪的代码，但显示了如何

then()

如果我们想增加对解决Promise的支持，我们只需要修改代码以

callback

then()

    if (succeeded) {      // If your callback returned a value,      // resolve the returned Promise to it...      if (typeof result.then === 'function') {        // ...unless it is a Promise itself,        // in which case we just pass our internal        // resolve and reject to then() of that Promise        result.then(resolve, reject)      } else {        resolve(result)      }    } else {      // If your callback threw an error,      // reject the returned Promise with it      reject(error)    }  })

让我再次澄清，这不是一个实际的Promise实现，并且有很多漏洞和不兼容之处。但是，它应该使您直观地了解Promise库如何实现解析为Promise。在您对这个想法感到满意之后，我建议您看看实际的Promise实现如何处理此问题。