JavaScript Promises

What are Promises?

• Similar to callback functions…however they are MUCH cleaner. If you were to use callback functions for every condition, you’d be in callback hell.

Use Cases:

• Tell code what to do when an action succeeds (resolve)
• Tell code what to do when an action fails (reject)
• Chaining | A promise chain will add a .then statement allowing for cleaner code

Syntax of creating a promise…

let wakeUp = new Promise((resolve, reject) => {
let a = "awake"
if (a == "awake"){
  resolve('Good Morning!')
}else{
  reject('It is time to wake up!')}
})
console.log(wakeUp)
// 
this logs...
Promise {<fulfilled>: "Good Morning!"}
__proto__: Promise
[[PromiseState]]: "fulfilled"
[[PromiseResult]]: "Good Morning!"

Traditional callback functions may look like this…

doSomething(function(result) {
  doSomethingElse(result, function(newResult) {
    doThirdThing(newResult, function(finalResult) {
      console.log('Got the final result: ' + finalResult);
    }, failureCallback);
  }, failureCallback);
}, failureCallback);

But a promise chain can look like this…

doSomething()
.then(function(result) {
  return doSomethingElse(result);
})
.then(function(newResult) {
  return doThirdThing(newResult);
})
.then(function(finalResult) {
  console.log('Got the final result: ' + finalResult);
})
.catch(failureCallback);

Learn more about promises on the MDN Webiste: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Using_promises