What's the difference between returning value or Promise.resolve from then()

Question

What is the difference between:

new Promise(function(res, rej) {      res("aaa");    })    .then(function(result) {      return "bbb";    })    .then(function(result) {      console.log(result);    });

and this:

new Promise(function(res, rej) {      res("aaa");    })    .then(function(result) {      return Promise.resolve("bbb");    })    .then(function(result) {      console.log(result);    });

I'm asking as I'm getting different behaviour Using Angular and $http service with chaining .then(). A bit too much code hence first the example above.

Answer 1

In simple terms, inside a then handler function:

A) When x is a value (number, string, etc):

1. return x is equivalent to return Promise.resolve(x)
2. throw x is equivalent to return Promise.reject(x)

B) When x is a Promise that is already settled (not pending anymore):

1. return x is equivalent to return Promise.resolve(x), if the Promise was already resolved.
2. return x is equivalent to return Promise.reject(x), if the Promise was already rejected.

C) When x is a Promise that is pending:

1. return x will return a pending Promise, and it will be evaluated on the subsequent then.

Read more on this topic on the Promise.prototype.then() docs.

Answer 2

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.

Quoting from the Promises/A+ spec:

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.

The key thing to notice here is this line:

if x is a promise, adopt its state [3.4]

link: https://promisesaplus.com/#point-49