I Do Async Programming

And so can you (I promise)

   Sarah Zinger

            @SarahsZingers

var self

class SarahZinger {
    constructor() {
        super();
        this._age = 0;
        this._knowsJavascript = false;
    }
    get isHappy() {
        return this._knowsJavascript;
    }
    changeWorld() {
        this._age >=25 ?
            this.socialWork():
            this.code();
    }
    birthday() {
        this._age++;
    }
}
    

What's Asynchronous Programming?

1

2

3

Synchronous Single Threaded

Multi Threaded

Asynchronous Single Threaded

so what?

1

2

3

The Dream: Synchronous Single Threaded

Asynchronous Single Threaded

The Reality: Synchronous Single Threaded

What's a Callback?

Javascript allows for asynchronous calls

that work something like this

Callbacks Don't Seem So bad?

TweetDatabase.getTweet(function(err, tweet){
  
    tweet.getAllReTweets(function(err, retweets){

        retweets.getUsers(function(err, users){

              users[0].follow();
        })
    })
})
    

WhY, you no bubble?

try {
    TweetDatabase.getTweet(function(err, tweet){
      
        tweet.getAllReTweets(function(err, retweets){
    
            retweets.getUsers(function(err, users){
    
                  users[0].follow();
            })
        })
    })
}
catch(err) {
    console.log("nested errors do not bubble! :(")
}    

The dark Side of Callbacks

TweetDatabase.getTweet(function(err, tweet){
    if(err){
        console.log("we found an err", err)
    }   
    tweet.getAllReTweets(function(err, retweets){
        if(err){
            console.log("we found an err", err)
        }
        retweets.getUsers(function(err, users){
            if(err){
                console.log("we found an err", err)
            }
            users[0].follow();
        })
    })
})
    

Wouldn't It Be Nice...

//inside a bunch of nested callbacks:
var users = retweets.getUsers();

//later...
users[0].follow();

//later...
users.addAllToList()
    

With PRomises, You Can.

var promisedUsers = retweets.getUsers();

// later...

promisedUsers.then(function(users){
    users.followAll();
})

// later... 
promisedUsers.then(function(users) {
    users.addToList();
})

    

WHY NO NESTED CALLBACKS THO?

var promisedUsers = Q.promise(function (success, failure) {
    retweets.getUsers(function(err, users){
        if(err) {
            failure(err)
        } else {
            success(users)
        }
    })
}

.then() to save the Day

TweetDatabase.getTweet()
.then(function(tweets){
    return tweet.getAllReTweets();
})
.then(function(retweets){
    return retweets.getUsers();
})
.then(function(users){
    users.followAll();
})
.catch(function(err){
    console.log("we found an err", err)
})

    

Don't Do this

TweetDatabase.getTweet()
.then(function(tweets){
    tweet.getAllReTweets()
    .then(function(retweets){
        retweets.getUsers()
            .then(function(users){
                users.followAll();
            })
    })

})
.catch(function(err){
    console.log("we found an err", err)
})

    

Do Keep LEarning

You're Missing The Point Of Promises

We Have a Problem With Promises

Promise Anti-patterns

Q

Sarah Zinger

sarahzinger.com

         SarahsZingers