Все, что вам нужно знать о обещании. Все

Promise.all([Promise1, Promise2, Promise3])
 .then(result) => {
   console.log(result)
 })
 .catch(error => console.log(`Error in promises ${error}`))

// A simple promise that resolves after a given time
const timeOut = (t) => {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      resolve(`Completed in ${t}`)
    }, t)
  })
}

// Resolving a normal promise.
timeOut(1000)
 .then(result => console.log(result)) // Completed in 1000

// Promise.all
Promise.all([timeOut(1000), timeOut(2000)])
 .then(result => console.log(result)) // ["Completed in 1000", "Completed in 2000"]

// A simple promise that resolves after a given time
const timeOut = (t) => {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      resolve(`Completed in ${t}`)
    }, t)
  })
}

const durations = [1000, 2000, 3000]

const promises = []

durations.map((duration) => {
  // In the below line, two things happen.
  // 1. We are calling the async function (timeout()). So at this point the async function has started and enters the 'pending' state.
  // 2. We are pushing the pending promise to an array.
  promises.push(timeOut(duration)) 
})

console.log(promises) // [ Promise { "pending" }, Promise { "pending" }, Promise { "pending" } ]

// We are passing an array of pending promises to Promise.all
// Promise.all will wait till all the promises get resolves and then the same gets resolved.
Promise.all(promises)
.then(response => console.log(response)) // ["Completed in 1000", "Completed in 2000", "Completed in 3000"]

// A simple promise that resolves after a given time
const timeOut = (t) => {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      if (t === 2000) {
        reject(`Rejected in ${t}`)
      } else {
        resolve(`Completed in ${t}`)
      }
    }, t)
  })
}

const durations = [1000, 2000, 3000]

const promises = []

durations.map((duration) => {
  promises.push(timeOut(duration)) 
})

// We are passing an array of pending promises to Promise.all
Promise.all(promises)
.then(response => console.log(response)) // Promise.all cannot be resolved, as one of the promises passed got rejected.
.catch(error => console.log(`Error in executing ${error}`)) // Promise.all throws an error.

const durations = [1000, 2000, 3000]

promises = durations.map((duration) => {
  return timeOut(duration).catch(e => e) // Handling the error for each promise.
})

Promise.all(promises)
  .then(response => console.log(response)) // ["Completed in 1000", "Rejected in 2000", "Completed in 3000"]
  .catch(error => console.log(`Error in executing ${error}`))
view raw

for (let i=0;i<50000; i += 1) {
 sendMailForUser(user[i]) // Async operation to send a email
}

// Async function to send mail to a list of users.
const sendMailForUsers = async (users) => {
  const usersLength = users.length
  
  for (let i = 0; i < usersLength; i += 100) { 
    const requests = users.slice(i, i + 100).map((user) => { // The batch size is 100. We are processing in a set of 100 users.
      return triggerMailForUser(user) // Async function to send the mail.
       .catch(e => console.log(`Error in sending email for ${user} - ${e}`)) // Catch the error if something goes wrong. So that it won't block the loop.
    })
    
    // requests will have 100 or less pending promises. 
    // Promise.all will wait till all the promises got resolves and then take the next 100.
    await Promise.all(requests)
     .catch(e => console.log(`Error in sending email for the batch ${i} - ${e}`)) // Catch the error.
  }
}


sendMailForUsers(userLists)

// Function to fetch Github info of a user.
const fetchGithubInfo = async (url) => {
  console.log(`Fetching ${url}`)
  const githubInfo = await axios(url) // API call to get user info from Github.
  return {
    name: githubInfo.data.name,
    bio: githubInfo.data.bio,
    repos: githubInfo.data.public_repos
  }
}

// Iterates all users and returns their Github info.
const fetchUserInfo = async (names) => {
  const requests = names.map((name) => {
    const url = `https://api.github.com/users/${name}`
    return fetchGithubInfo(url) // Async function that fetches the user info.
     .then((a) => {
      return a // Returns the user info.
      })
  })
  return Promise.all(requests) // Waiting for all the requests to get resolved.
}


fetchUserInfo(['sindresorhus', 'yyx990803', 'gaearon'])
 .then(a => console.log(JSON.stringify(a)))

/*
Output:
[{
  "name": "Sindre Sorhus",
  "bio": "Full-Time Open-Sourcerer ·· Maker ·· Into Swift and Node.js ",
  "repos": 996
}, {
  "name": "Evan You",
  "bio": "Creator of @vuejs, previously @meteor & @google",
  "repos": 151
}, {
  "name": "Dan Abramov",
  "bio": "Working on @reactjs. Co-author of Redux and Create React App. Building tools for humans.",
  "repos": 232
}]
*/