promise.js

// PROMISES!
// There are two ways to work with promises.

// Option 1 the easiest once you understand it.
// JavaScript has a feature called Async-Await
// Notice the *async* keyword before function
async function asyncPattern() {
  // In async functions, you can get the result of
  // promises without .then. Just stick a 'await'
  // in front of it and it will wait for the promise
  // to get it's result.
  const four = await promiseAdd(2, 2)
  console.log('async 2 + 2 = ' + four)
  
  // You can do await <promise> as many times as you'd like.
  const eight = await promiseAdd(4, 4)
  console.log('async 4 + 4 = ' + eight)
}

// Note:
// async functions return promises too. 


// Option 2
promiseAdd(2, 2).then((four)=>{
  // When the promise finishes, the result comes in as
  // the first argument (in this case, four)
  console.log('2 + 2 = ' + four)
}).catch((error)=>{
  // If an error occurs in the promise, this will fire
  console.log(error)
})

// Important note:
// Do NOT call .then in a .then function. It is a quick
// way to get a bug.

promiseAdd(2, 2).then((four)=>{
  // Instead, return your promise inside of the 'then'
  // funcition like so:
  
  return promiseAdd(four, four) // essentially 4 + 4
  
  // Returning a value from a promise creates another
  // promise, to which you can call .then again.
}).then((eight)=>{
  // The result of the previous promise comes in like
  // normal.
  console.log('4 + 4 = ' + eight)
}).catch((error)=>{
  // If an error occurs in any of the promises, it will
  // be given here.
  console.log(error)
})