Object literal (hash) with Promise.all

Question

I have a situation where it would be quite convenient to use Promise.all like so Promise.all({}) instead of the more standard Promise.all([]).

but this doesn't seem to work

Promise.all({a:1,b:2}).then(function(val){
   console.log('val:',val);
});

whilst this does of course

Promise.all([1,2,3]).then(function(val){
   console.log('val:',val);
});

(what I would expect would be for Promise.all to map the values of the Object literal, but leave the keys intact.)

But the MDN docs for Promise seem to indicate that Promise all will work for any iterable. To my knowledge, an object literal {} is an iterable. So what am I missing?

Answer 1

Here is another async / await ES6 solution:

async function allOf(hash = {}) {
  const promises = Object.keys(hash).map(async key => ({[key]: await hash[key]}));
  const resolved = await Promise.all(promises);
  return resolved.reduce((hash, part) => ({...hash, ...part}), {});
}

This converts the keys into a promise that produces a single element hash. Then at the end we combine all the hashes in the array to a single hash. You could compact this to a one-liner even, at the cost of readability.

async function allOfOneLiner(hash = {}) {
  return (await Promise.all(Object.keys(hash).map(async k => ({[k]: await hash[k]})))).reduce((h, p) => ({...h, ...p}), {});
}

Answer 2

Object does not have an Iterator symbol if you look at the mdn documentation for those.

What you can do, is use a tool function to create an object iterable and later consume it.

reference to objectEntries source, however nodejs does not implement Reflect, so for the purpose of using it with node I just change it into using Object.keys()

function objectEntries(obj) {
    let index = 0;

    // In ES6, you can use strings or symbols as property keys,
    // Reflect.ownKeys() retrieves both
    let propKeys = Object.keys(obj);

    return {
        [Symbol.iterator]() {
            return this;
        },
        next() {
            if (index < propKeys.length) {
                let key = propKeys[index];
                index++;
                return { value: [key, obj[key]] };
            } else {
                return { done: true };
            }
        }
    };
}

Answer 3

Use Object.values. Works in Firefox Nightly:

Promise.all(Object.values({a:1,b:2}))
.then(vals => console.log('vals: ' + vals)) // vals: 1,2
.catch(e => console.log(e));

var console = { log: msg => div.innerHTML += msg + "<br>" };

<div id="div"></div>

Then, to put the results back in an object, we can make a Promise.allParams function:

Promise.allParams = o => 
  Promise.all(Object.values(o)).then(promises =>
    Object.keys(o).reduce((o2, key, i) => (o2[key] = promises[i], o2), {}));

// Demo:

Promise.allParams({a:1,b:2}).then(function(val){
   console.log('val: ' + JSON.stringify(val)); // val: {"a":1,"b":2}
});

var console = { log: msg => div.innerHTML += msg + "<br>" };

<div id="div"></div>