Are Promise.resolve and new Promise(resolve) interchangeable

Question

I think Promise.resolve and new Promise(resolve) are interchangeable.

Consider this:

A.

new RSVP.Promise(function (resolve, reject) {
    resolve();
}).then(function () {
    return new RSVP.Promise(function (resolve) {
        resolve("HI")
    });
}).then(function (result) {
    console.log(result);
});

B.

new RSVP.Promise(function (resolve, reject) {
    resolve();
}).then(function () {
    return RSVP.resolve("HI");
}).then(function (result) {
    console.log(result);
});

Both print "HI" as I expected.

So I think if I don't need to "reject" anything. I can just write RSVP.resolve(); for simplicity.

But consider this example:

new RSVP.Promise(function (resolve, reject) {
    resolve();
}).then(function () {
    return new RSVP.Promise(function (resolve, reject) {
        setTimeout(function () {
            resolve("HI")
        }, 3000);
    });
}).then(function (result) {
    console.log(result);
});

How can I use RSVP.resolve(); to replace? I tried for example:

new RSVP.Promise(function (resolve, reject) {
    resolve();
}).then(function () {
    return setTimeout(function () {
        return new RSVP.resolve("HI");
    }, 3000);
}).then(function (result) {
    console.log(result);
});

This prints something else instead of "HI". So is it possible to use RSVP.resolve(); here? Are these two interchangeable?

Answer 1

First and foremost

I think Promise.resolve and new Promise(resolve) are interchangeable.

Nope. Promise.resolve will create a promise which is already resolved, whereas new Promise(resolve) creates a promise which is neither resolved nor rejected.

---

In the last example,

return setTimeout(function () {
    return new RSVP.resolve("HI");
}, 3000);

means that, you are returning the result of setTimeout function, not a promise object. So, the current then handler will return a resolved promise with the result of setTimeout. That is why you are seeing a weird object.

---

In your particular case, you want to introduce a delay before resolving the promise. It is not possible with Promise.resolve. The penultimate method you have shown in the question, is the way to go.