README.md

value-or-promise

A thenable to streamline a possibly sync / possibly async workflow.

Installation

yarn add value-or-promise or npm install value-or-promise

Basic Motivation

Instead of writing:

function myFunction() {
    const valueOrPromise = getValueOrPromise();

    if (isPromise(valueOrPromise)) {
        return valueOrPromise.then(v => onValue(v));
    }
    
    return onValue(valueOrPromise);
}

...write:

function myFunction() {
    return new ValueOrPromise(getValueOrPromise)
        .then(v => onValue(v))
        .resolve();
}

When working with functions that may or may not return promises, we usually have to duplicate handlers in both the synchronous and asynchronous code paths. In the most basic scenario included above, using value-or-promise already provides some code savings, i.e. we only have to reference doSomethingWithValue once.

More Chaining

Things start to get even more beneficial when we add more sync-or-async functions to the chain.

Instead of writing:

function myFunction() {
    const valueOrPromise = getValueOrPromise();

    if (isPromise(valueOrPromise)) {
        return valueOrPromise
            .then(v => first(v))
            .then(v => second(v));
    }

    const nextValueOrPromise = first(ValueOrPromise)

    if (isPromise(nextValueOrPromise)) {
        return nextValueOrPromise.then(v => second(v));
    }
    
    return second(nextValueOrPromise);
}

...write:

function myFunction() {
    return new ValueOrPromise(getValueOrPromise)
        .then(v => first(v))
        .then(v => second(v))
        .resolve();
}

Error Handling

Even with shorter chains, value-or-promise comes in handy when managing errors.

Instead of writing:

function myFunction() {
    try {
        const valueOrPromise = getValueOrPromise();

        if (isPromise(valueOrPromise)) {
            return valueOrPromise
                .then(v => onValue(v))
                .catch(error => console.log(error));
        }
    
        const nextValueOrPromise = onValue(valueOrPromise);

        if (isPromise(nextValueOrPromise)) {
            return nextValueOrPromise
                .catch(error => console.log(error));
        }

        return nextValueOrPromise;
    } catch (error) {
        console.log(error);
    }
}

...write:

function myFunction() {
    return new ValueOrPromise(getValueOrPromise)
        .then(v => onValue(v))
        .catch(error => console.log(error))
        .resolve();
}

Alternatives

A simpler way of streamlining the above is to always return a promise.

Instead of writing:

function myFunction() {
    const valueOrPromise = getValueOrPromise();

    if (isPromise(valueOrPromise)) {
        return valueOrPromise.then(v => onValue(v));
    }
    
    return onValue(valueOrPromise);
}

...or writing:

function myFunction() {
    return new ValueOrPromise(getValueOrPromise)
        .then(v => onValue(v))
        .resolve();
}

...we could write:

function myFunction() {
    return Promise.resolve(getValueOrPromise)
        .then(v => onValue(v));
}

...but then we would always have to return a promise! If we are trying to avoid the event loop when possible, this will not suffice.

ValueOrPromise.all(...)?

We can use ValueOrPromise.all(...) analogous to Promise.all(...) to create a new ValueOrPromise object that will either resolve to an array of values, if none of the passed ValueOrPromise objects contain underlying promises, or to a new promise, if one or more of the ValueOrPromise objects contain an underlying promise, where the new promise will resolve when all of the potential promises have resolved.

For example:

function myFunction() {
    const first = new ValueOrPromise(getFirst);
    const second = new ValueOrPromise(getSecond);
    return ValueOrPromise.all([first, second]).then(
        all => onAll(all)
    ).resolve();
}

myFunction with return a value if and only if getFirst and getSecond both return values. If either returns a promise, myFunction will return a promise. If both getFirst and getSecond return promises, the new promise returned by myFunction will resolve only after both promises resolve, just like with Promise.all.

Inspiration

The value-to-promise concept is by Ivan Goncharov.

Implementation errors are my own.