Can an asynchronous function potentially return an object with a "then" function included?

Question

Can an asynchronous function potentially return an object with a "then" function included?

When using the JavaScript async function (or a regular function that returns a Promise), any object with a function containing a "then" field is considered a Promise. However, is it possible to have such an object be the resolved value of a Promise?

For example, when calling the function await f(), no result is returned. If the field name in then is different, then a result is actually returned.


async function f() {
    return {
        then() {
            //This function simply has a fitting name as "then".
            return 42;
        }
    };
}

async function main() {
    let result = await f(); // execution stops here.
    console.log(result);
    console.log(result.then());
}
main();

javascript asynchronous promise

Answer 1

Answer №1

Upon delving into the information provided by await, a noticeable pattern emerges

await expression

expression

An object of type Promise, a thenable entity, or any value poised for anticipation.

Further exploration in the realm of thenable brings to light that

A thenable boasts an implementation of the .then() functionality

and

To facilitate seamless integration with prevailing Promise structures, the programming language sanctions the utilization of thenables instead of promises.

Hence, when employing await and furnishing an item furnished with a then capability, it signals to the JavaScript engine that this is indicative of a thenable object. Consequently, it expects said object to adhere to the prescribed thenable protocol.

Answer 2

Upon delving into the information provided by await, a noticeable pattern emerges

await expression

expression

An object of type Promise, a thenable entity, or any value poised for anticipation.

Further exploration in the realm of thenable brings to light that

A thenable boasts an implementation of the .then() functionality

and

To facilitate seamless integration with prevailing Promise structures, the programming language sanctions the utilization of thenables instead of promises.

Hence, when employing await and furnishing an item furnished with a then capability, it signals to the JavaScript engine that this is indicative of a thenable object. Consequently, it expects said object to adhere to the prescribed thenable protocol.

Answer 3

Answer №2

When working with an async function, it will always return a Promise. This means that you are essentially returning a Promise which in turn returns an object with a .then method. However, a proper .then method requires two arguments, both of which should be functions - onFulfilled and onRejected. Therefore, your current implementation of the then function is not considered a valid Promise.

A more effective approach would look something like this:

async function f() {
    return {
        then(onFulfilled, onRejected) {
            onFulfilled(42);
        }
    };
}
async function main() {
    let result = await f();
    console.log(result);
    f().then(result => console.log(result));
}
main();

Answer 4

When working with an async function, it will always return a Promise. This means that you are essentially returning a Promise which in turn returns an object with a .then method. However, a proper .then method requires two arguments, both of which should be functions - onFulfilled and onRejected. Therefore, your current implementation of the then function is not considered a valid Promise.

A more effective approach would look something like this:

async function f() {
    return {
        then(onFulfilled, onRejected) {
            onFulfilled(42);
        }
    };
}
async function main() {
    let result = await f();
    console.log(result);
    f().then(result => console.log(result));
}
main();

Can an asynchronous function potentially return an object with a "then" function included?

Answer №1

Answer №2

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