This can be handy if you have a rate limit on API requests or if you need to pass the result of each promise to the next one. 

function fetchMessages(username) {    return fetch(`https://example.com/api/messages/${username}`)        .then(response => response.json());}function getUsername(person) {    return person.username;}async function chainedFetchMessages(p, username) {    // In this function, p is a promise. We wait for it to finish,    // then run fetchMessages().    const obj  = await p;    const data = await fetchMessages(username);    return { ...obj, [username]: data};}const msgObj = peopleArr    .map(getUsername)    .reduce(chainedFetchMessages, Promise.resolve({}))    .then(console.log);// ⦘ {glestrade: [ … ], mholmes: [ … ], iadler: [ … ]}