Promises and async await

A promise represents a value that may be available later.

It can be:

• pending,
• fulfilled,
• rejected.

Promise example

fetch('/api/users')
  .then((response) => response.json())
  .then((users) => console.log(users))
  .catch((error) => console.error(error))

async await

async/await is cleaner syntax for working with promises.

async function loadUsers() {
  try {
    const response = await fetch('/api/users')
    const users = await response.json()
    console.log(users)
  } catch (error) {
    console.error(error)
  }
}

await pauses the async function, not the entire JavaScript runtime.

Common mistake

Do not forget to handle rejected promises.

Bad:

async function loadUsers() {
  const response = await fetch('/api/users')
  return response.json()
}

Better:

async function loadUsers() {
  try {
    const response = await fetch('/api/users')
    return await response.json()
  } catch (error) {
    throw new Error('Failed to load users')
  }
}

Interview answer

A promise represents the eventual result of an asynchronous operation. It can be pending, fulfilled, or rejected. async/await is syntax built on promises that makes asynchronous code easier to read. Rejections should be handled with .catch or try/catch.