Promise

A Promise represents the eventual completion (or failure) of an async operation.

States

• pending — initial state
• fulfilled — operation succeeded
• rejected — operation failed

Creating a Promise

const promise = new Promise((resolve, reject) => {
  setTimeout(() => {
    const success = true
    if (success) {
      resolve('Data loaded!')
    } else {
      reject(new Error('Something went wrong'))
    }
  }, 1000)
})

Consuming Promises

promise
  .then(result => console.log(result))  // 'Data loaded!'
  .catch(error => console.error(error))
  .finally(() => console.log('Done'))   // always runs

Promise.all

const p1 = fetch('/api/users')
const p2 = fetch('/api/posts')
const p3 = fetch('/api/comments')

Promise.all([p1, p2, p3])
  .then(([users, posts, comments]) => {
    // all three resolved
  })
  .catch(err => {
    // any one rejected
  })

Promise.allSettled

Promise.allSettled([p1, p2, p3])
  .then(results => {
    results.forEach(result => {
      if (result.status === 'fulfilled') {
        console.log(result.value)
      } else {
        console.log(result.reason)
      }
    })
  })