How to detect if a user is online

Many browsers have implemented events “online” and “offline”; in addition to this, browsers also expose the navigator.onLine property that will return “online” or “offline”

How many browsers is many?

caniuse says every major browser (Chrome, Firefox, Safair, IE11…) except Opera mini has this functionality (as of July 4th 2018).

How can I check the online status?

> navigator.onLine

How can I listen to events?


Be loud if core functionality depends on staying online

If your app depends on autosaving,


Be like workflowy*. Tell the users, make them listen.


You will likely need more error handling than this. For example: what if your server is offline? what if the request body has malformed? what if the server is up but your database is down? what if the api hits a 404 and do you let your devs know about this?

* referral link

This post is not about Backbone

We’re gonna talk about video games.

Imagine you’re playing Mario Kart on a four person split screen. There are four Yoshis on the screen for each player. How many Yoshis should we model?

The obvious answer is four: there are four Yoshis on the screen, so there must be four Yoshis. The problem is that this answer misses something important, and fails to answer a critical question: what if there were no Yoshis on anyone’s screen right now? Does that mean there should be zero Yoshis? No, of course not, Yoshi can fall behind in the race and should still exist.

The real answer is there is only one Yoshi – one source of truth that dictates where Yoshi is, how fast he’s going, what items he has, and what he’s about to do. Fundamentally, all this means is that Yoshi exists separate from his representation to the user. We can organize our code to represent this by grouping our data about Yoshi in a single location.

Mario Kart on Super Nintendo
Mario Kart on Super Nintendo


Models as the source of truth

Models are used to encapsulate data. A Model stores data properties and serves as the ultimate reference in other parts of our game. In our example Yoshi, Mario, Bowser are all Models – they have a single state and source of truth that’s associated with them. With this distinction we avoid searching for information on multiple Yoshis or creating logic to determine which copy of Yoshi is the most updated version of our racer.

Models are isolated from the code that displays the screen to the user so while Yoshi may turn a corner and disappear from the screen, our game logic continues figuring out how fast and far Yoshi has gone. We decide what to show users through building Views.


Views as Lakitu

Imagine all the Lakitus filming the race

Views decide what to show the user. Each View is associated with a model, and decides exactly how that model should be shown on screen. We wouldn’t want to just show the user all the stats about Yoshi, we want to make it a cute green dinosaur in a cart that you can see and aim at – View’s make that happen.

You can have lots of Views for a single model. For instance, even though there’s one Yoshi there’s actually up to four Views associated with him! In regards to Views, four is actually the right answer. Our Model for Yoshi has only one location and one speed, but our View shows him in four different places on each screen and renders him going in different directions depending on where you 

If we began to store information about Yoshi in the graphic, we run the risk of creating data inconsistent with our original source of truth. What if there are multiple Yoshi graphics? By separating the storage of data from the representation of data we ensure that all players see the same Yoshi.

Events as a Blue Shell

When Bowser grabs a blue shell and throws it, this is a global event that every racer wants to know about. A smart group of Views will have event listeners on its respective Models. Smart Views can listen to events from the Model such as a ‘change’ or ‘add’ event. As our Models pass these event notifications to our Views, the screen responds by showing the player a blue shell rushing towards first place.

Our method of drawing data becomes increasingly more important as we add online players to the race from Mario Karts game servers. Having one source of data ensures that every player has a uniform experience. As there is only one Yoshi, there should only be one tweet, wall post and chat message that our Views are listening to.

Libraries and Frameworks

Concise code lends itself to become more reusable and testable. Our clear sense of separation and organization allows us to write modular code that is focused on clearly defined tasks. Every library and framework implements this differently and it will take some exploring to find the right fit.

Who is the keyword “this”?

`this` – NBA Edition

The keyword `this` is a difficult javascript topic that has been debated and pondered over for centuries – more or less. Its meaning is ambiguous at first glance mainly because what the keyword `this` refers to changes depend on where it is called. Here we explore some of the various ways in which this will be changed throughout your use and we will use the NBA as the backdrop to the conversation. * Feel free to open your console in Chrome and follow along with the examples. The global scope is the NBA. If we open up our console in Chrome and type this, Chromes returns back the window object – or the NBA in our context. The same occurs with console.log(this).

Free Floating Functions belong to the NBA

function whoIsThis(){

whoIsThis(); // returns current context; 

When we call a free floating function like this, the keyword `this` is assigned to the context in which its called. If we type whoIsThis() in our console, `this` refers to the our global window scope. If we tried this in server side code, the keyword this refers to an internal node.js object.

Players and Methods use this Lets create Kevin Durant. Kevin has a method that references our whoIsThis function from above.

var kevinDurant = {
    shoot3pct: 0.391,
    jersey: "Durant",
    findOutWhoIsThis: whoIsThis

If a function is invoked as a method of a player/object the keyword `this` now refers to the player. Type `kevinDurant.findOutWhoIsThis()` in the console and the function returns the object! Remember, keyword `this` is set at run time. Using the same whoIsThis function above, `this` is now Kevin Durant and not the NBA.

Steal moves with Apply and Call Lets create a function to shoot 3 pointers

var shoot3PT = function() {
    if (this.shoot3pct === undefined) { 
        return "Error: NBA has no 3pt percentage";
    if (Math.random() > this.shoot3pct) {
        return "3 points for "+ this.jersey +"!";
    } else {
        return "Shot missed, try again!";

Try calling `shoot3PT()` in the console. Error! The first argument of the apply or call functions will set the context of the function. Since KD has a shoot3pct property, we will simulate a three point shot by Durant with these functions.

    // or 
    // both simulate a 3 pointer

Nightmare mode: Remember we can always refer to `shoot3PT` as a method on Kevin Durant. Imagine another player, say Stephen Curry, wants to also use that function. We could use `apply` to set the context to him.

    kevinDurant.shoot3PT = shoot3PT

    stephenCurry = {
       this.shoot3pct: 0.99,
       this.jersey: "Curry"

    // Here we call apply on a method 
    // from another Player object!


Stephen Curry is the context for the shoot3PT function because of apply. Think of the possibilities –
We can simulate an entire basketball game with Javascript!

Stay tuned: next week we will explore the Game of Thrones edition.