How to map an array of strings for a Promise.all argument?

Question

What I am looking to do is to use an array of strings to use those values to map a new array of promise functions for a Promise.all argument.

I think explains my thought process so far, as well as the issue I am running into.

const strings = ['A', 'B', 'C'] // Varies in size
const stringFunctions = strings.map(item => {
  const func = () => {
    promised(item) // Returns Promised Boolean
  }
  return func
})

const PromiseAll = new Promise(function(resolve, reject) {
  Promise.all(stringFunctions)
    .then(items => {
      const obj = {};
      items.forEach((item, index) => {
        obj[strings[index]] = item;
      });
      resolve(obj); 
      // returns { A: func(), B: func(), C: func() }
      // expected { A: bool, B: bool, C: bool }
    })
    .catch(error => {
      reject(error);
    });
}

func has a statement block but does not return anything. You need a return in there: return promised(item) — trincot
– trincot, Commented Aug 19, 2018 at 19:14
Promise.all does not take an array of promise functions, it takes an array of promises? — Bergi
– Bergi, Commented Aug 19, 2018 at 19:21

danh · Accepted Answer · 2018-08-19 19:18:35Z

2

This can be done without any explicit promise creation (you have all you need in the promised function and in Promise.all()).

let strings = [ ... ]
let promises = strings.map(string => promised(string));
Promise.all(promises).then(results => {
    // results is a new array of results corresponding to the "promised" results
});

answered Aug 19, 2018 at 19:18

danh

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4 Comments

Patrick Roberts Over a year ago

Also if promised() is not a variadic function and takes exactly one argument, you can simplify that to Promise.all(strings.map(promised)).then(results => { ... })

ms_nitrogen Over a year ago

If I wanted to use the Promise.all for an export, I assume I'd name a const P = promise.all(...); export default P? I'm looking for the exported object results more than the promise itself.

danh Over a year ago

@ms_nitrogen - export something that is evaluated synchronously when the module loads (like a function or a class definition). It can provide an async function that the importer then invokes.

ms_nitrogen Over a year ago

@danh I wrapped it all up for a callback function, and it worked perfectly! Cheers!

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