A promise can be returned from a then() method of a promise.

You can then call its then() method, creating a chain of promises.

I think the best way to explain this concept is to use fetch(), a JavaScript feature we’ll introduce later on in details during this masterclass.

In short, we use fetch() to get a resource from the network.

In this example, we call fetch() to get JSON from GitHub with my user details (try with yours) using this URL: https://api.github.com/users/flaviocopes

fetch('https://api.github.com/users/flaviocopes')

fetch() returns a promise which resolves to a Response object. When we got that, we need to call its json() method to get the body of the response:

fetch('https://api.github.com/users/flaviocopes')
  .then(response => response.json())

This in turn returns a promise, so we can add another .then() with a callback function that is called when the JSON processing has finished, with the job of writing the data we got to the console:

fetch('https://api.github.com/users/flaviocopes')
  .then(response => response.json())
  .then(data => {
    console.log('User data', data)
  })

For comparison, with async functions we’d write the above code in this way:

const getData = async () => {
  const response = await fetch('https://api.github.com/users/flaviocopes')
  const data = await response.json()
  console.log('User data', data)
}

getData()

Lessons in this unit: