Obj Flatten, then Reduce Promises

script.js

const obj = { "foo": { "bar": "0", "pew": "hell world" }, "quux": { "baz": "1", "world": "wonderful" }};

const flatObj = Object.keys(obj).reduce((retObj, key) => Object.assign({},
  retObj,
  Object.keys(obj[key]).reduce((deepRetObj, deepKey) => Object.assign({},
    deepRetObj,
    { [`${key}.${deepKey}`]: obj[key][deepKey] }
  ), {})
), {});

console.log(JSON.stringify(flatObj));

Object.keys(flatObj).reduce((prevProm, key) => (
  prevProm.then((prevVal) =>
    new Promise(
      resolve => { setTimeout(() => { console.log(key); resolve(key); }, 1000); }
    )
  )
), Promise.resolve()).then(
  (output) => { console.info(JSON.stringify(output)); },
  (errors) => { console.error(JSON.stringify(errors)); }
);

Sample output:

{"foo.bar":"0","foo.pew":"hell world","quux.baz":"1","quux.world":"wonderful"}
foo.bar
foo.pew
quux.baz
quux.world
"quux.world"
[Finished in 4.143s]