9-promises.md

Promises

You need to know what these are! They will save you from "callback hell".

In a previous life, you might have written the following asynchronous code.

setTimeout(function oneSecondLater () {
  console.log(1);
  setTimeout(function twoSecondsLater () {
    setTimeout(function threeSecondsLater () {
      console.log(3);
      setTimeout(function fourSecondsLater () {
        console.log('4: do you see the problem?')
      }, 1000);
    }, 1000);
    console.log(2);
  }, 1000);
}, 1000);

Promises are abstract structures that help deal with asynchronous code.

// Usually, your promise library provides a function like this.
function delay (millis) {
  // Promise is a global available in modern JS
  // All promises are constructed with an immediate executed function
  // It should call resolve() with the value to pass down the chain
  // It should call reject() with an Error if something went wrong
  return new Promise((resolve, reject) => {
    setTimeout(resolve, millis)
  })
}

// This is an easy way to start a promise chain.
Promise.resolve()
	.then(function () {
	  // return promises in then() to make the chain wait for it
	  return delay(1000)
	})
	.then(() => 1) // promises pass their "return" values on
	.then((n) => console.log(n))
	.then(() => delay(1000))
	.then(() => console.log(2))
	.then(() => delay(1000))
	.then(() => console.log(3))
	.then(() => delay(1000))
	.then(() => console.log('4: much better'))
	.catch((err) => console.error(err))

Promises are perfect for things like XHR (replace jQuery AJAX).

// assume got is a global request tool
got('http://ip.jsontest.com', {json: true})
	.then((response) => console.log(`ip: ${response.body.ip}`))