Background

I am trying to learn promises, and I have a promise chain I want to improve on.

Problem

While learning how to chain promises, I fail to see why anyone would rather return a promise instead of returning it's value.

Take the following example, which uses promise chaining:

let myObj = new MyClass();

myObj.getInfo()
    .then(result => writeOutput(FILE_NAME, result))
    .then(console.log(FILE_NAME + " plete"))
    .catch(error => console.error(error));

class MyClass{

    getInfo() {
        return new Promise(function(fulfil, reject) {
            fulfill("I like bananas");
        });
}

Here I have to chain 2 times. But if I were to directly return the result from the method getInfo() instead of returning a Promise I could potentially do something like the following:

let myObj = new MyClass();

let str = myObj.getInfo();

writeOutput(FILE_NAME, str)
    .then(console.log(FILE_NAME + " plete"))
    .catch(error => console.error(error));

Questions

So as you can see I am a little confused.

1. Given that getInfo() is in fact async, is it possible to achieve a similar code to the one in my second code sample?
2. If it were possible, would it be a good idea? How would you do it?

Background

I am trying to learn promises, and I have a promise chain I want to improve on.

Problem

While learning how to chain promises, I fail to see why anyone would rather return a promise instead of returning it's value.

Take the following example, which uses promise chaining:

let myObj = new MyClass();

myObj.getInfo()
    .then(result => writeOutput(FILE_NAME, result))
    .then(console.log(FILE_NAME + " plete"))
    .catch(error => console.error(error));

class MyClass{

    getInfo() {
        return new Promise(function(fulfil, reject) {
            fulfill("I like bananas");
        });
}

Here I have to chain 2 times. But if I were to directly return the result from the method getInfo() instead of returning a Promise I could potentially do something like the following:

let myObj = new MyClass();

let str = myObj.getInfo();

writeOutput(FILE_NAME, str)
    .then(console.log(FILE_NAME + " plete"))
    .catch(error => console.error(error));

Questions

So as you can see I am a little confused.

1. Given that getInfo() is in fact async, is it possible to achieve a similar code to the one in my second code sample?
2. If it were possible, would it be a good idea? How would you do it?

• javascript
• node.js
• promise

4 Answers 4

You can only return a value from some function if that value is immediately available when that function is called (on the same tick of the event loop). Remember that return is synchronous.

If it is not available right away then you can only return a promise (or you can use a callback but here you are specifically asking about promises).

For a more detailed explanation see this answer that I wrote some time ago to a question asking on how to return a result of an AJAX call from some function. I explained why you cannot return the value but you can return a promise:

• jQuery: Return data after ajax call success

Here is another related answer - it got downvoted for some reason but I think it explains a similar issue that you're asking about her:

• Return value in function from a promise block

I fail to see why anyone would rather return a promise instead of returning it's value.

Because you don't have the value yet.

Given that getInfo() is in fact async, is it possible to achieve a similar code to the one in my second code sample?

If it's asynchronous, it must return a promise.

A syntax that avoids then calls (but still uses and produces promises) is made possible by async/await:

async function writeInfo(FILE_NAME) {
    const myObj = new MyClass();
    try {
        const str = await myObj.getInfo();
        await writeOutput(FILE_NAME, str);
        console.log(FILE_NAME + " plete");
    } catch (error) {
        console.error(error);
    }
}
writeInfo("…");

The purpose of a promise is to say: "The data will be there sometime, just continue with your coding for now", so that your page won't get stuck for example. Basically you are using Promises for example to load something from the server.

Another way of looking at the first question ("why would anyone..."):

When you call .then or .catch on a promise, the pending promise returned from the call is appended to the end of an existing promise chain. (Allowing the "existing chain" might only contain one promise).

When you resolve (not "fulfill", you can't fulfill a promise with a promise) a promise with a promise, the promise used in resolution is inserted at the head of what remains of the promise chain.

The promise used in resolution must be fulfilled with a value before the next .then or .catch clause in the chain is called.