Medium - Promise.all Examples

Promise-all.js

// Example 1:
const dog = new Promise((resolve, reject) => {
  setTimeout(() => resolve('🐶'), 1000)
})
const cat = new Promise((resolve, reject) => {
  setTimeout(() => resolve('🐈'), 2000)
})

Promise.all([dog, cat]).then((values) => {
  // Order of values will be in the same order 
  // in which promises are present in the array
  console.log(values) // ['🐶', '🐈']
})

// Example 2:
const bear = new Promise((resolve, reject) => {
  setTimeout(() => reject('🐻'), 1000)
})
const panda = new Promise((resolve, reject) => {
  setTimeout(() => resolve('🐼'), 2000)
})

Promise.all([bear, panda])
  .then((values) => {
    console.log(values)
  })
  .catch((error) => {
    console.error(error) // 🐻
  })

// Practical Usage:
// This would be useful in the case where 
// you want to fetch data from multiple resources 
// and then consolidate them to form a response 
// before sending it back to the client.
Promise.all([
    fetch('/endpoint0'),
    fetch('/endpoint1'),
    fetch('/endpoint2'),
]).then(response => console.log(response))
.catch(error => console.log(error))