#olin.js #4 — homework

Due Tuesday Feb 5 before class

We're gonna make a shitty Twitter.

Reading

Sessions

A few classes ago, we showed you how to use a database. Now we're going to introduce the concept of sessions. A session is a data store that is unique to each user's web browser, but the information is only visible to the server. Think of it as giving each user a unique order number, and when they request a webpage, you can customize it to their liking—but if they lose their order number, that information is gone. Thus, it's generally only appropriate to store some kinds of data in a session, like what account the user is logged in as or temporary settings. Depending on the user's setup, the next time they close their browser the session may be cleared.

Let's compare the types of ways we can store information.

1. Sessions. A session is any temporary information about the person requesting a webpage. If a user logs in, you can add their username to their session. If they come back in a week and haven't cleared their cookies, they will still be logged in. If they clear their cookies, next time they go to your webpage, they start a new session.
2. Database. A database is for long-term data storage you never want deleted (unintentionally). You would store a user's username in a session, but store their name, date of birth, and dreams in a database. That way their information outlives them closing their browser window.

For completeness sake, let's consider other ways we can explore data:

1. Cookies. Cookies are strings of information you store on a user's computer. Unlike sessions, cookies are generally in plaintext, i.e. a user can go ahead and edit their cookies. Cookies would be useful to store a user's preferred font size, but not their username, lest someone evil go ahead and set document.cookie = 'username=supersecretadmindad'. Muy mal.
2. Global variables. You might hear them described as "bad" or "evil" but our only consideration is that global variables go away when your server resets. On Heroku, your server might reset as often as every 10 minutes! Don't expect global variables to last long.
3. Files. Though we haven't covered it yet, Node can read and write files on your computer. Heroku doesn't let you write files, however, so we are going to remain willfully ignorant for a while.

In order to enable sessions in Express you have to do the following

app.configure(function(){
  ...
  app.use(express.cookieParser(process.env.SECRET || 'fake_secret'));
  app.use(express.session());
  app.use(app.router);
  app.use(express.static(path.join(__dirname, 'public')));
});

Note that app.router MUST go after you set up sessions or else it won't work. Let's take a look at what's going on here

• express.cookieParser is a middleware (glue code). We're not storing the userID that is logged in on our cookies (at least not in plaintext). We're encrypting it with a secret key that we're passing along. The client will use this cookie so that our server can reference the proper session. Think of it as a key/value store where the encrypted cookie is our key and our session data is our value. If this is confusing, read this on StackOverflow.
• express.session is what actually tells express that we'll be using sessions

Or as a shortcut, you can tell Express to set up a new app with sessions pre-enabled

express --sessions

Alright, now that we've set up our session, we want to be able to store stuff in sessions (such as things about the logged in user). We can do this with req.sessions. So in pesudo code you want something like

app.get('/login', function (req, res){
  user = new User
  user.save(function (){
    req.sessions.user = user;
  });
})

Now if the user goes to another page, we can just get the same user from our req.session variable. In pesudocode

app.get('/mystuff', function(req, res){
  var user = req.sessions.user;
  // do stuff here
});

The setInterval function

Let's imagine for a moment that we want a notice to show up on our website every second. Remember how we said that JS is async? This means that you can't just tell the process to pause for a second every time (well you can but this would be bad because then your app hangs for a second).

However, you can use the setInterval function. Open up Chrome and paste this into your Chrome JS console

var alertMsg = function(){ alert('im cooool'); };
setInterval(alertMsg, 1000);

setInteval takes a function to execute as it's first parameter and the interval to execute it at (in ms) as it's second parameter.

Check out this article on javascript timing for more details on how setInterval works.

Assignment

Paul and David are going to Twitter next year. Let's see just how easy it is to do their job.

• Fork this repo to your own account
• Generate an Express app
• npm install all the packages you think you'll need and add them to your package.json file
• Create an app that has the following routes
  • GET / => shows a list of all the recent tweets. This page uses the javascript setTimeout to poll for new tweets every 2 seconds. This means that new tweets should stream in without having to refresh the page.
  • GET /users/new => shows a form that allows someone to input in a name for their user account. When they hit submit it should POST to /users/new. If a user with that name doesn't exist, it should create a new user, and redirect to /users/:user. If a user with that name exists, it should log them in (with sessions).
  • GET /users/:user => shows the user a list of their tweets. If the user is logged in, it also shows a form that they can use to create a new tweet. When they hit the "tweet" button it should POST to /tweets/:user via an AJAX $.post call, if that post is successful, this tweet should also show up on their list of tweets without them refreshing the page. If that tweet fails (as in it's longer than 140 characters), an error message should be displayed to the user.
  • Do some funky CSS stuff so that the pages look like the following (Don't use Twitter Bootstrap. That's cheating.)
• Create a Heroku Procfile like the one we made for you in previous homeworks
• Push to Heroku and add the url to the homework sheet