The rule is, if the function that is in the then handler returns a value, the promise resolves/rejects with that value, and if the function returns a promise, what happens is, the next then clause will be the then clause of the promise the function returned, so, in this case, the first example falls through the normal sequence of the thens and prints out values as one might expect, in the second example, the promise object that gets returned when you do Promise.resolve("bbb")'s then is the then that gets invoked when chaining(for all intents and purposes). The way it actually works is described below in more detail.

引用承诺/A+规范:

The promise resolution procedure is an abstract operation taking as input a promise and a value, which we denote as [[Resolve]](promise, x). If x is a thenable, it attempts to make promise adopt the state of x, under the assumption that x behaves at least somewhat like a promise. Otherwise, it fulfills promise with the value x. This treatment of thenables allows promise implementations to interoperate, as long as they expose a Promises/A+-compliant then method. It also allows Promises/A+ implementations to “assimilate” nonconformant implementations with reasonable then methods.

这里需要注意的关键是这一行:

如果x是一个承诺，采用它的状态[3.4] 链接:https://promisesaplus.com/ # point-49

你已经得到了一个很好的正式答案。我想我应该加一个简短的。

以下几点与Promises/A+ Promises相同:

要求的承诺。resolve(在Angular中是$q.when) 调用承诺构造函数并在其解析器中解析。在你的例子中，这是新的q。 从then回调返回一个值。 要求的承诺。都在一个有值的数组上，然后提取那个值。

因此，对于承诺值或普通值X，下面这些都是相同的:

Promise.resolve(x);
new Promise(function(resolve, reject){ resolve(x); });
Promise.resolve().then(function(){ return x; });
Promise.all([x]).then(function(arr){ return arr[0]; });

毫无疑问，承诺规范是基于承诺解决过程的，它支持库之间的简单互操作(如$q和本机承诺)，并使您的生活总体上更简单。无论何时出现承诺解决方案，解决方案都会产生整体一致性。

The rule is, if the function that is in the then handler returns a value, the promise resolves/rejects with that value, and if the function returns a promise, what happens is, the next then clause will be the then clause of the promise the function returned, so, in this case, the first example falls through the normal sequence of the thens and prints out values as one might expect, in the second example, the promise object that gets returned when you do Promise.resolve("bbb")'s then is the then that gets invoked when chaining(for all intents and purposes). The way it actually works is described below in more detail.

引用承诺/A+规范:

The promise resolution procedure is an abstract operation taking as input a promise and a value, which we denote as [[Resolve]](promise, x). If x is a thenable, it attempts to make promise adopt the state of x, under the assumption that x behaves at least somewhat like a promise. Otherwise, it fulfills promise with the value x. This treatment of thenables allows promise implementations to interoperate, as long as they expose a Promises/A+-compliant then method. It also allows Promises/A+ implementations to “assimilate” nonconformant implementations with reasonable then methods.

这里需要注意的关键是这一行:

如果x是一个承诺，采用它的状态[3.4] 链接:https://promisesaplus.com/ # point-49

只是一点。在调试器中，'resolve'函数不会返回。如果是最后一个语句，则返回。'Resolve'将把承诺设置为已完成，但如果下面有更多语句，则继续执行。

你可以在这里读到:“为什么javascript ES6承诺在解析后继续执行?”

这让我很困惑，因为在大多数例子中都没有解释。所以从现在开始，我必须记住使用'return resolve(value)'或'if resolve(value) else…其他代码'或只是使用'resolve'作为最后的语句。

唯一的区别是，当您返回promise .resolve("bbb")时，您创建了一个不必要的承诺。从oncompleted()处理程序返回承诺将启动承诺解析。这就是承诺链的工作原理。

这两个例子的行为应该是一样的。

在then()处理程序中返回的值将成为then()返回的承诺的解析值。如果.then中返回的值是一个promise，那么then()返回的promise将“采用该promise的状态”，并像返回的promise一样解决/拒绝。

在第一个示例中，在第一个then()处理程序中返回“bbb”，因此“bbb”被传递到下一个then()处理程序。

在第二个示例中，返回一个承诺，该承诺立即用值"bbb"进行解析，因此"bbb"被传递到下一个then()处理程序。这里的Promise.resolve()是无关的。

结果是一样的。

如果你能给我们举个例子，它能表现出不同的行为，我们就能告诉你为什么会这样。