JavaScript array .reduce with async/await

The problem is that your accumulator values are promises - they're return values of async functions. To get sequential evaluation (and all but the last iteration to be awaited at all), you need to use

const data = await array.reduce(async (accumP, current, index) => {
  const accum = await accumP;
  …
}, Promise.resolve(…));

That said, for async/await I would in general recommend to use plain loops instead of array iteration methods, they're more performant and often simpler.

I like Bergi's answer, I think it's the right way to go.

I'd also like to mention a library of mine, called Awaity.js

Which lets you effortlessly use functions like reduce, map & filter with async / await:

import reduce from 'awaity/reduce';

const posts = await reduce([1,2,3], async (posts, id) => {

  const res = await fetch('/api/posts/' + id);
  const post = await res.json();

  return {
    ...posts,
    [id]: post
  };
}, {})

posts // { 1: { ... }, 2: { ... }, 3: { ... } }