Retrieve tuples from Promise results / async functions

promise-tuple.js

/**
 * Returns a Promise which resolves with a value in form of a tuple.
 * @param promiseFn A Promise to resolve as a tuple.
 * @returns Promise A Promise which resolves to a tuple of [error, ...results]
 */
export function tuple (promise) {
  return promise
    .then((...results) => [null, ...results])
    .catch(error => [error])
}

/**
 * Returns a function which creates a tuple-ful Promise.
 * @param fn A function to create a tuple from.
 * @returns A function which creates a tuple from `fn`.
 */
export function tuplify (fn) {
  return function tupleFn (...args) {
    return tuple(fn(...args))
  }
}

usage.js

import { tuple, tuplify } from './promise-tuple'

// Using tuple to pass a Promise to be resolved as a tuple.
async function doFetch () {
  const [error, result] = await tuple(fetch('https://google.com'))
  
  if (error) {
    return console.log('Oh, no!', error)
  }
  
  console.log('Yey!', result)
}

// Using tuplify to retrieve tuples from async function.
const tupleFetch = tuplify(fetch)

async function doTupleFetch () {
  const [error, result] = await tupleFetch('https://google.com')
  
  if (error) {
    return console.log('Oh, no!', error)
  }
  
  console.log('Yey!', result)
}