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Node.js Essential Training
Illustration by Richard Downs

Emitting events and attaching listeners


From:

Node.js Essential Training

with Joseph LeBlanc

Video: Emitting events and attaching listeners

One of the benefits of JavaScript is that event driven programming is built into the language. Node has an event emitter you can use to broadcast events to all listeners. In this video, we'll emit some events in the middle of an existing application. First, let's go to the exercise files. Go to chapter nine, video one, and then copy the start folder onto the desktop. Now lets install the dependencies. Go to terminal, and change directory to that folder. I'm just going to drag it in to get the path.
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  1. 3m 9s
    1. Welcome
      42s
    2. What you should know before watching this course
      57s
    3. Using the exercise files
      47s
    4. Using the challenges
      43s
  2. 18m 50s
    1. Why use Node.js?
      2m 1s
    2. Choosing a Node.js installation process
      4m 56s
    3. Installing Node.js on the Mac
      2m 5s
    4. Installing Node.js on Windows
      1m 43s
    5. Installing Node.js via Node Version Manager on Ubuntu Linux
      4m 45s
    6. Installing Node.js via Node Version Manager on a Mac
      3m 20s
  3. 13m 54s
    1. Exploring language additions to the V8 JavaScript engine
      3m 38s
    2. Understanding require() and modules
      6m 39s
    3. Understanding callback execution in the event loop
      3m 37s
  4. 12m 23s
    1. Initializing Node.js projects
      2m 37s
    2. Finding an NPM
      3m 30s
    3. Maintaining projects using the npm command
      6m 16s
  5. 21m 4s
    1. Creating modules with getters and setters
      6m 33s
    2. Understanding module caching and scopes
      6m 15s
    3. Implementing JavaScript creational patterns
      5m 8s
    4. Challenge: Leveraging module caching
      55s
    5. Solution: Leveraging module caching
      2m 13s
  6. 32m 27s
    1. Starting Express applications
      3m 42s
    2. Handling GET requests returning JSON
      3m 26s
    3. Navigating large datasets using request variables in routes
      5m 4s
    4. Modifying data through PUT requests
      4m 32s
    5. Supplying middleware to modify HTTP response headers
      4m 37s
    6. Generating HTML views in Jade
      8m 9s
    7. Challenge: List all records as JSON
      37s
    8. Solution: List all records as JSON
      2m 20s
  7. 25m 37s
    1. Evaluating unit testing methodologies
      3m 8s
    2. Isolating existing code for testing
      6m 47s
    3. Supplying helpers for test data
      2m 35s
    4. Writing unit tests with Mocha
      6m 5s
    5. Simulating HTTP requests with SuperTest
      7m 2s
  8. 28m 45s
    1. Connecting to MongoDB via Mongoose
      4m 16s
    2. Defining schemas for collections in Mongoose
      3m 47s
    3. Querying MongoDB collections
      5m 32s
    4. Persisting Express sessions through MongoDB and mongo-connect
      7m 6s
    5. Logging in to Express applications using Passport
      8m 4s
  9. 21m 47s
    1. Controlling the built-in REPL
      4m 18s
    2. Constructing a custom REPL
      5m 40s
    3. Receiving command-line arguments with Optimist
      6m 14s
    4. Building command-line tools
      3m 35s
    5. Challenge: Launch a custom REPL from a command
      41s
    6. Solution: Launch a custom REPL from a command
      1m 19s
  10. 15m 46s
    1. Emitting events and attaching listeners
      5m 32s
    2. Streaming chunked data through readable streams
      2m 33s
    3. Controlling readable streams
      1m 52s
    4. Piping readable data into writable streams
      2m 10s
    5. Handling duplex streams over TCP
      3m 39s
  11. 4m 6s
    1. Deploying projects to cloud hosting services
      2m 36s
    2. Finding Node.js resources
      1m 30s

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Node.js Essential Training
3h 17m Intermediate Oct 16, 2013

Viewers: in countries Watching now:

Node.js is a powerful tool for controlling web servers, building applications, and creating event-driven programming. And it brings JavaScript—a language familiar to all web developers—into an environment independent of web browsers. Learn all about Node.js and start creating JavaScript applications in this course with Joseph LeBlanc. He shows how to install Node.js on Mac, Windows, and Linux and dives deep into its modules and Express framework for app development. Learn how to unit test your code, start sessions on web servers, stream data, and create simple command-line tools.

Topics include:
  • Why use Node.js?
  • Installing Node.js
  • Understanding the event loop
  • Initializing Node.js projects
  • Creating modules with getters and setters
  • Starting Express applications
  • Testing your code
  • Working with sessions and databases
  • Building command-line tools
  • Emitting events and attaching listeners
  • Controlling readable streams
Subjects:
Developer Servers Cloud Computing Programming Languages
Software:
JavaScript Node.js
Author:
Joseph LeBlanc

Emitting events and attaching listeners

One of the benefits of JavaScript is that event driven programming is built into the language. Node has an event emitter you can use to broadcast events to all listeners. In this video, we'll emit some events in the middle of an existing application. First, let's go to the exercise files. Go to chapter nine, video one, and then copy the start folder onto the desktop. Now lets install the dependencies. Go to terminal, and change directory to that folder. I'm just going to drag it in to get the path.

Then press Return, and then type npm install. Now let's have a look at the code. Now before continuing, make sure that the connection string in db.js is set to connect to your Mongo DB database. For more information, have a look at the chapter on Mongo DB. I'm going to go to routes and index.js. Currently we have a route for marking flights as arrived. It connects to the Mongo DB database, creates a document, then sends back either a success message or failure status.

Let's consider the following scenario. We may have a web application, where we really don't care about what's happening in the database. This could be more of a non-critical application, where we're simply making a best effort to mark the arrival times. In this case, the speed of our application is more important than making sure the record properly records in Mongo DB. Instead of waiting on Mongo DB to finish inserting the document, we can emit an event and let any number of listeners handle that data. In the meantime, we send a successful status code back to the browser and then handle recording everything else separately.

So let's do that now. First, let's add an event emitter. To do that, I'm going to go to the top of this page. And I'm going to define a new variable called Emitter with a capital E. And to get that, I'm going to require the events module. This is a core module in Node.js, so we don't have to install it. And then I'm going to get the event emitter property from that module. Next, I'm going to create a new emitter. To do that, I'm going to create a new variable called flight emitter, and I'm just going to set it to a new instance of emitter.

Now let's go back down to the function where we're handling the arrivals. So just after marking a flight as being arrived, I'm going to emit an event. I'm going to use that flight emitter that we just created, and I'm going to call the emit method. The first argument is going to be the event type. I'm going to call this an arrival. And then the second argument is the data that we want to pass in this event. I want to pass the flight itself. And now, finally, I want to send the success status right away.

So now, let's add some listeners for this arrival event. The first listener is going to record the data in the database. So I'm going to cut everything from line 44. And it looks like we have an extra response here so I'm just going to remove it. And now, I'm going to go back up to the top of the page and I'm going to create a listener. So to do that, I'm going to call flightEmitter. And then I'm going to call the on method. So now I'm going to create a listener for an arrival event. I'm going to call flight emitter, and I'm going to use the on method.

And then the first argument I'm going to pass is the name of the event, which is arrival. And the second argument I'm going to pass is a function. That function is going to take the flight as it's single argument. So now I'm going to paste in all of that database code. And now I need to make some adjustments. Since we're having the flight passed in right here, I don't need to look it up in an object or an array. So, I'm just going to use that flight object directly. And then finally, I don't need to send any responses. So, I'm going to remove that. And I'm also going to remove this entire else clause.

The only thing that we want to do is log an error on the console if we get one from the database. Let's add another listener. Again I'm going to use flightEmitter. And I'm going to call the on method, and again we're going to listen for arrival events. And in this listener, I'm just going to put the flight number out on the console. So I'll just say flight arrived, and then concatenate the number onto the end of that. Let's go to Terminal and start up the server.

Type in node server and then press Return. Now let's go to the browser and let's fire up Post Man. So in this case, let's mark flight 567 as arrived. So I'm making a put request to localhost at port 3000, slash flight, slash 567, slash arrived. And I am going to send this. It says status success. And then here on the command line, we got that Flight arrived: 567. Now let's go to the arrivals page. Go to localhost at port 3000 slash arrivals.

So we have the arrivals that were marked from before, and the one that we marked just now. And we can also mark another flight as arrived. I'm going to do Flight 577 and then just send that. Successful. It's marked that flight has arrived, 577. If I go back to the arrivals page and hit Refresh, it's now going to list that in the arrivals as well. Event emitters make it possible to send a piece of data to any number of listeners. Since the Functions execute asynchronously, you can emit Events and continue the rest of the code without waiting on the listeners to finish.

In the next video, we'll listen to events on a stream.

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