Javascript - how to compose asynchronous functions

Question

I have this code:

const compose2 = (f, g) => (...args) => f(g(...args))
const compose = (...fns) => fns.reduce(compose2)

const count = arr => arr.length
const split = str => str.split(/\s+/)
const addAsyncWord = async str => `${str} some words obtained asynchronously`
const read = text => text


const word = compose(
        count,
        split,
        (await addAsyncWord),
        read
    )

console.log('word ->', await word()) //<- edited

but is throwing the following error:

Uncaught SyntaxError: missing ) in parenthetical

output expected after reading, concat words (obtained from a server randomly, needs to be asynchronously), spliting words, and finally count the total length, must be something similar to this:

word -> 10

How to make this to work? thanks

From a purely functional agnle you can't. Function composition is based on a functor (of the function type). If you have to deal with async functions you need another type, namely continuations and the appropriate tools to compose them (applicative and monad). — user5536315
– user5536315, Commented Nov 20, 2021 at 21:46
Take a look at ramda implementation of composeWith and andThen: github.com/ramda/ramda/blob/master/source — Khalt
– Khalt, Commented Nov 20, 2021 at 22:13
I'm not getting a syntax error from your code? But word is a function that you never call… — Bergi
– Bergi, Commented Nov 21, 2021 at 0:37
You'll need const compose2 = (f, g) => async (...args) => f(await g(...args)). An await in the arguments list of compose won't achieve anything, addAsyncWord is a function not a promise that can be waited for. — Bergi
– Bergi, Commented Nov 21, 2021 at 0:39

user2580401 · Accepted Answer · 2021-11-21 18:00:49Z

4

I will post my solution here if helps anyone in the future:

const compose2 = (f, g) => async (...args) => f(await g(...args))
const compose = (...fns) => fns.reduce(compose2)

const count = arr => arr.length
const split = str => str.split(/\s+/)
const addAsyncWord = async str => `${str} some words obtained asynchronously`
const read = text => text

const word = compose(
    count,
    split,
    addAsyncWord,
    read
)

const test = async () => {
    const word1 = await word('Hello world')
    console.log('word ->', word1)
}

test()

thank you for your comments.

answered Nov 21, 2021 at 18:00

user2580401

1,9099 gold badges28 silver badges36 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

DevTheJo Over a year ago

if you have more than one argument it could be rewrited as const compose2 = (f, g) => async (...args) => f(...await g(...args))

Collectives™ on Stack Overflow

Javascript - how to compose asynchronous functions

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related