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

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.

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.

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

Both of your examples should behave pretty much the same.

A value returned inside a then() handler becomes the resolution value of the promise returned from that then(). If the value returned inside the .then is a promise, the promise returned by then() will "adopt the state" of that promise and resolve/reject just as the returned promise does.

In your first example, you return "bbb" in the first then() handler, so "bbb" is passed into the next then() handler.

In your second example, you return a promise that is immediately resolved with the value "bbb", so "bbb" is passed into the next then() handler. (The Promise.resolve() here is extraneous).

The outcome is the same.

If you can show us an example that actually exhibits different behavior, we can tell you why that is happening.

You already got a good formal answer. I figured I should add a short one.

The following things are identical with Promises/A+ promises:

• Calling Promise.resolve (In your Angular case that's $q.when)
• Calling the promise constructor and resolving in its resolver. In your case that's new $q.
• Returning a value from a then callback.
• Calling Promise.all on an array with a value and then extract that value.

So the following are all identical for a promise or plain value X:

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]; });

And it's no surprise, the promises specification is based on the Promise Resolution Procedure which enables easy interoperation between libraries (like $q and native promises) and makes your life overall easier. Whenever a promise resolution might occur a resolution occurs creating overall consistency.

The only difference is that you're creating an unnecessary promise when you do return Promise.resolve("bbb"). Returning a promise from an onFulfilled() handler kicks off promise resolution. That's how promise chaining works.

Just a point. The 'resolve' function does not return as you can see in a debugger. It seems to return if it is the last statement. 'Resolve' will set the promise as fullfilled but continue execution if there are more statements below.

As you can read here: 'Why does javascript ES6 Promises continue execution after a resolve?'

This was very confusing for me because it is not explained in most examples. So from now on, I have to remember to use 'return resolve(value)' or an 'if resolve(value) else ...other code' or just use 'resolve' as the last statement.