Promises/Callbacks

Promises represent the completion of an asynchronous function. They can be used as an alternative to chaining functions.

let doSecond = () => {
  console.log('Do second.')
}

let doFirst = new Promise((resolve, reject) => {
  setTimeout(() => {
    console.log('Do first.')

    resolve()
  }, 500)
})

doFirst.then(doSecond)

Example using promises

function makeRequest(method, url) {
  return new Promise((resolve, reject) => {
    let request = new XMLHttpRequest()

    request.open(method, url)
    request.onload = resolve
    request.onerror = reject
    request.send()
  })
}

makeRequest('GET', 'https://url.json')
  .then((event) => {
    console.log(event.target.response)
  })
  .catch((err) => {
    throw new Error(err)
  })