# javascript

## Promise
> https://medium.com/javascript-scene/master-the-javascript-interview-what-is-a-promise-27fc71e77261

>https://youtu.be/DHvZLI7Db8E - web dev simplified - JavaScript Promises In 10 Minutes

A promise is an object which can be returned synchronously from an asynchronous function. It will be in one of 3 possible states:

* Fulfilled: onFulfilled() will be called (e.g., resolve() was called)
* Rejected: onRejected() will be called (e.g., reject() was called)
* Pending: not yet fulfilled or rejected

Making some requests using **callbacks** -

function watchTutorialCallback(callback, errorCallback) {
  let userLeft = false
  let userWatchingCatMeme = false

  if (userLeft) {
    errorCallback({
      name: 'User Left', 
      message: ':('
    })
  } else if (userWatchingCatMeme) {
    errorCallback({
      name: 'User Watching Cat Meme',
      message: 'WebDevSimplified < Cat' 
    })
  } else {
    callback('Thumbs up and Subscribe')
  }
}

watchTutorialCallback(message => {
  console.log(message)
}, error => {
  console.log(error.name + ' ' + error.message)
})

*callback hell*

Making the same requests using **promises** -

function watchTutorialPromise() {
  let userLeft = false
  let userWatchingCatMeme = false
  return new Promise((resolve, reject) => {
    if (userLeft) {
      reject({
        name: 'User Left', 
        message: ':('
      })
    } else if (userWatchingCatMeme) {
      reject({
        name: 'User Watching Cat Meme',
        message: 'WebDevSimplified < Cat' 
      })
    } else {
      resolve('Thumbs up and Subscribe')
    }
  })
}

watchTutorialPromise().then(message => {
  console.log(message)
}).catch(error => {
  console.log(error.name + ' ' + error.message)
})

Because .then() always returns a new promise, it’s possible to chain promises with precise control over how and where errors are handled.