Understanding the this keyword in JavaScript

Understanding the this keyword in JavaScript

In this article, we are going to learn about Javascript keyword “this” and how the value of “this” is assigned in different scenarios.

In this article, we are going to learn about Javascript keyword “this” and how the value of “this” is assigned in different scenarios.

It’s probably safe to say that the this keyword is one of the most misunderstood parts of JavaScript. Admittedly, I used to throw the this keyword around until my script worked and it confused the hell out of me (and still confuses many other JS developers). It wasn’t until I learned about lexical scope, how functions are invoked, scope context, and a few context changing methods that I really understood it.

Before you dive into this article, here’s a few very important points to takeaway and remember about the this keyword:

  • The this keyword’s value has nothing to do with the function itself, how the function is called determines the this value
  • It can be dynamic, based on how the function is called
  • You can change the this context through .call().apply() and .bind()

Table of Contents

  1. Default this context
  2. 1.1. Window Object, global scope
  3. 1.2. Object literals
  4. 1.3. Prototypes and Constructors
  5. 1.4. Events
  6. Dynamic this
  7. Changing this context
  8. 3.1. Using .call(), .apply() and .bind()
  9. 3.2. forEach scoping
  10. 3.3. .bind()
  11. “Jumping scope”
  12. 4.1. Important note: this and the arguments Object are the only objects that don’t follow the rules of lexical scope.
  13. 4.2. jQuery $(this)

Default this context

There are a few different ways the this value changes, and as we know it’s usually the call-site that creates the context.

Window Object, global scope

Let’s take a quick example at how simply calling regular functions binds the thisvalue differently:

What can we expect the this value to be? By default, this should always be the window Object, which refers to the root – the global scope. So when we console.log(this); from our function, as it’s invoked by the window (simply just called), we should expect the this value to be our window Object:

// define a function
var myFunction = function () {
  console.log(this); // [object Window]
};

// call it
myFunction();

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Object literals

Inside Object literals, the this value will always refer to its own Object. Nice and simple to remember. That is good news when invoking our functions, and one of the reasons I adopt patterns such as the module pattern for organising my objects.

Here’s how that might look:

// create an object
var myObject = {};

// create a method on our object
myObject.someMethod = function () {
  console.log(this);
};

// call our method
myObject.someMethod();

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Here, our window Object didn’t invoke the function – our Object did, so this will refer to the Object that called it:

// create an object
var myObject = {};

// create a method on our object
myObject.someMethod = function () {
  console.log(this); // myObject
};

// call our method
myObject.someMethod();

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Prototypes and Constructors

The same applies with Constructors:

var myConstructor = function () {
    this.someMethod = function () {
        console.log(this);
    };
};

var a = new myConstructor();
a.someMethod();

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And we can add a Prototype Object as well:

var myConstructor = function () {
    this.someMethod = function () {
        console.log(this);
    };
};

myConstructor.prototype = {
    somePrototypeMethod: function () {
        console.log(this);
    }
};

var a = new myConstructor();
a.someMethod();
a.somePrototypeMethod();

c4ca4238a0b923820dcc509a6f75849b.js hosted with ❤ by GitHub

Interestingly, in both cases the this value will refer to the Constructor object, which will be myConstructor.

Events

When we bind events, the same rule applies, the this value points to the owner. The owner in the following example would be the element.

// let's assume .elem is <div class="elem"></div>
var element = document.querySelector('.elem');
var someMethod = function () {
  console.log(this);
};
element.addEventListener('click', someMethod, false);

c4ca4238a0b923820dcc509a6f75849b.js hosted with ❤ by GitHub

Here, this would refer to <div class="elem"></div>.

Dynamic this

The second point I made in the intro paragraph was that this is dynamic, which means the value could change. Here’s a real simple example to show that:

// let's assume .elem is <div class="elem"></div>
var element = document.querySelector('.elem');

// our function
var someMethod = function () {
  console.log(this);
};

// when clicked, `this` will become the element
element.addEventListener('click', someMethod, false); // <div>

// if we just invoke the function, `this` becomes the window object
someMethod(); // [object Window]

c4ca4238a0b923820dcc509a6f75849b.js hosted with ❤ by GitHub

Changing this context

There are often many reasons why we need to change the context of a function, and thankfully we have a few methods at our disposal, these being .call().apply() and .bind().

Using any of the above will allow you to change the context of a function, which in effect will change the this value. You’ll use this when you want this to refer to something different than the scope it’s in.

Using .call().apply() and .bind()

You will often hear that “Functions are first class Objects,” which means that they can also have their own methods!

The .call() method allows you to change the scope with a specific syntax ref:

.call(thisArg[, arg1[, arg2[, ...]]]);

Usage would look something like this:

someMethod.call(anotherScope, arg1, arg1);

You’ll notice further arguments are all comma separated — this is the only difference between .call() and .apply():

someMethod.call(anotherScope, arg1, arg1); // commas
someMethod.apply(anotherScope, [arg1, arg1]); // array

With any of the above, they immediately invoke the function. Here’s an example:

var myFunction = function () {
  console.log(this);
};
myFunction.call();

Without any arguments, the function is just invoked and this will remain as the window Object.

Here’s a more practical usage, this script will always refer to the window Object:

var numbers = [{
  name: 'Mark'
},{
  name: 'Tom'
},{
  name: 'Travis'
}];
for (var i = 0; i < numbers.length; i++) {
  console.log(this); // window
}

The forEach method also has the same effect, it’s a function so it creates new scope:

var numbers = [{
  name: 'Mark'
},{
  name: 'Tom'
},{
  name: 'Travis'
}];
numbers.forEach(function () {
  console.log(this); // window
});

We could change each iteration’s scope to the current element’s value inside a regular for loop as well, and use this to access object properties:

var numbers = [{
  name: 'Mark'
},{
  name: 'Tom'
},{
  name: 'Travis'
}];
for (var i = 0; i < numbers.length; i++) {
  (function () {
    console.log(this.name); // Mark, Tom, Travis
  }).call(numbers[i]);
}

This is especially extensible when passing around other Objects that you might want to run through the exact same functions.

forEach scoping

Not many developers using forEach know that you can change the initial scope context via the second argument:

numbers.forEach(function () {
  console.log(this); // this = Array [{ name: 'Mark' },{ name: 'Tom' },{ name: 'Travis' }]
}, numbers); // BOOM, scope change!

Of course the above example doesn’t change the scope to how we want it, as it changes the functions scope for every iteration, not each individual one — though it has use cases for sure!

To get the ideal setup, we need:

var numbers = [{
  name: 'Mark'
},{
  name: 'Tom'
},{
  name: 'Travis'
}];
numbers.forEach(function (item) {
  (function () {
    console.log(this.name); // Mark, Tom, Travis
  }).call(item);
});

.bind()

Using .bind() is an ECMAScript 5 addition to JavaScript, which means it’s not supported in all browsers (but can be polyfilled so you’re all good if you need it). Bind has the same effect as .call(), but instead binds the function’s context prior to being invoked, this is essential to understand the difference. Using .bind() will not invoke the function, it just “sets it up”.

Here’s a really quick example of how you’d setup the context for a function, I’ve used .bind() to change the context of the function, which by default the this value would be the window Object.

var obj = {};
var someMethod = function () {
  console.log(this); // this = obj
}.bind(obj);
someMethod();

This is a really simple use case, they can also be used in event handlers as well to pass in some extra information without a needless anonymous function:

var obj = {};
var element = document.querySelector('.elem');
var someMethod = function () {
  console.log(this);
};
element.addEventListener('click', someMethod.bind(obj), false); // bind

“Jumping scope”

I call this jumping scope, but essentially it’s just some slang for accessing a lexical scope reference (also a bit easier to remember).

There are many times when we need to access lexical scope. Lexical scope is where variables and functions are still accessible to us in parent scopes.

var obj = {};

obj.myMethod = function () {
  console.log(this); // this = `obj`
};

obj.myMethod();

In the above scenario, this binds perfectly, but what happens when we introduce another function. How many times have you encountered a scope challenge when using a function such as setTimeout inside another function? It totally screws up any this reference:

var obj = {};
obj.myMethod = function () {
  console.log(this); // this = obj
    setTimeout(function () {
        console.log(this); // window object :O!!!
    }, 100);
};
obj.myMethod();

So what happened there? As we know, functions create scope, and setTimeout will be invoked by itself, defaulting to the windowObject, and thus making the this value a bit strange inside that function.

Important note: this and the arguments Object are the only objects that don’t follow the rules of lexical scope.

How can we fix it? There are a few options! If we’re using .bind(), it’s an easy fix, note the usage on the end of the function:

var obj = {};
obj.myMethod = function () {
  console.log(this); // this = obj
    setTimeout(function () {
        console.log(this); // this = obj
    }.bind(this), 100); // .bind() #ftw
};
obj.myMethod();

We can also use the jumping scope trick, var that = this;:

var obj = {};
obj.myMethod = function () {
  var that = this;
  console.log(this); // this = obj
    setTimeout(function () {
        console.log(that); // that (this) = obj
    }, 100);
};
obj.myMethod();

We’ve cut the this short and just simply pushed a reference of the scope into the new scope. It’s kind of cheating, but works wonders for “jumping scope”. With newcomers such as .bind(), this technique is sometimes frowned upon if used and abused.

One thing I dislike about .bind() is that you could end up with something like this:

var obj = {};
obj.myMethod = function () {
  console.log(this);
    setTimeout(function () {
        console.log(this);
        setTimeout(function () {
            console.log(this);
            setTimeout(function () {
                console.log(this);
                setTimeout(function () {
                    console.log(this);
                }.bind(this), 100); // bind
            }.bind(this), 100); // bind
        }.bind(this), 100); // bind
    }.bind(this), 100); // bind
};
obj.myMethod();

Do what makes sense!

jQuery $(this)

Yes, the same applies, don’t use $(this) unless you actually know what it’s doing. What it is doing is passing the normal this value into a new jQuery Object, which will then inherit all of jQuery’s prototypal methods (such as addClass), so you can instantly do this:

$('.elem').on('click', function () {
  $(this).addClass('active');
});

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JavaScript developers should you be using Web Workers?

JavaScript developers should you be using Web Workers?

Do you think JavaScript developers should be making more use of Web Workers to shift execution off of the main thread?

Originally published by David Gilbertson at https://medium.com

So, Web Workers. Those wonderful little critters that allow us to execute JavaScript off the main thread.

Also known as “no, you’re thinking of Service Workers”.

Photo by Caleb Jones on Unsplash

Before I get into the meat of the article, please sit for a lesson in how computers work:

Understood? Good.

For the red/green colourblind, let me explain. While a CPU is doing one thing, it can’t be doing another thing, which means you can’t sort a big array while a user scrolls the screen.

This is bad, if you have a big array and users with fingers.

Enter, Web Workers. These split open the atomic concept of a ‘CPU’ and allow us to think in terms of threads. We can use one thread to handle user-facing work like touch events and rendering the UI, and different threads to carry out all other work.

Check that out, the main thread is green the whole way through, ready to receive and respond to the gentle caress of a user.

You’re excited (I can tell), if we only have UI code on the main thread and all other code can go in a worker, things are going to be amazing (said the way Oprah would say it).

But cool your jets for just a moment, because websites are mostly about the UI — it’s why we have screens. And a lot of a user’s interactions with your site will be tapping on the screen, waiting for a response, reading, tapping, looking, reading, and so on.

So we can’t just say “here’s some JS that takes 20ms to run, chuck it on a thread”, we must think about where that execution time exists in the user’s world of tap, read, look, read, tap…

I like to boil this down to one specific question:

Is the user waiting anyway?

Imagine we have created some sort of git-repository-hosting website that shows all sorts of things about a repository. We have a cool feature called ‘issues’. A user can even click an ‘issues’ tab in our website to see a list of all issues relating to the repository. Groundbreaking!

When our users click this issues tab, the site is going to fetch the issue data, process it in some way — perhaps sort, or format dates, or work out which icon to show — then render the UI.

Inside the user’s computer, that’ll look exactly like this.

Look at that processing stage, locking up the main thread even though it has nothing to do with the UI! That’s terrible, in theory.

But think about what the human is actually doing at this point. They’re waiting for the common trio of network/process/render; just sittin’ around with less to do than the Bolivian Navy.

Because we care about our users, we show a loading indicator to let them know we’ve received their request and are working on it — putting the human in a ‘waiting’ state. Let’s add that to the diagram.

Now that we have a human in the picture, we can mix in a Web Worker and think about the impact it will have on their life:

Hmmm.

First thing to note is that we’re not doing anything in parallel. We need the data from the network before we process it, and we need to process the data before we can render the UI. The elapsed time doesn’t change.

(BTW, the time involved in moving data to a Web Worker and back is negligible: 1ms per 100 KB is a decent rule of thumb.)

So we can move work off the main thread and have a page that is responsive during that time, but to what end? If our user is sitting there looking at a spinner for 600ms, have we enriched their experience by having a responsive screen for the middle third?

No.

I’ve fudged these diagrams a little bit to make them the gorgeous specimens of graphic design that they are, but they’re not really to scale.

When responding to a user request, you’ll find that the network and DOM-manipulating part of any given task take much, much longer than the pure-JS data processing part.

I saw an article recently making the case that updating a Redux store was a good candidate for Web Workers because it’s not UI work (and non-UI work doesn’t belong on the main thread).

Chucking the data processing over to a worker thread sounds sensible, but the idea struck me as a little, umm, academic.

First, let’s split instances of ‘updating a store’ into two categories:

  1. Updating a store in response to a user interaction, then updating the UI in response to the data change
  2. Not that first one

If the first scenario, a user taps a button on the screen — perhaps to change the sort order of a list. The store updates, and this results in a re-rendering of the DOM (since that’s the point of a store).

Let me just delete one thing from the previous diagram:

In my experience, it is rare that the store-updating step goes beyond a few dozen milliseconds, and is generally followed by ten times that in DOM updating, layout, and paint. If I’ve got a site that’s taking longer than this, I’d be asking questions about why I have so much data in the browser and so much DOM, rather than on which thread I should do my processing.

So the question we’re faced with is the same one from above: the user tapped something on the screen, we’re going to work on that request for hopefully less than a second, why would we want to make the screen responsive during that time?

OK what about the second scenario, where a store update isn’t in response to a user interaction? Performing an auto-save, for example — there’s nothing more annoying than an app becoming unresponsive doing something you didn’t ask it to do.

Actually there’s heaps of things more annoying than that. Teens, for example.

Anyhoo, if you’re doing an auto-save and taking 100ms to process data client-side before sending it off to a server, then you should absolutely use a Web Worker.

In fact, any ‘background’ task that the user hasn’t asked for, or isn’t waiting for, is a good candidate for moving to a Web Worker.

The matter of value

Complexity is expensive, and implementing Web Workers ain’t cheap.

If you’re using a bundler — and you are — you’ll have a lot of reading to do, and probably npm packages to install. If you’ve got a create-react-app app, prepare to eject (and put aside two days twice a year to update 30 different packages when the next version of Babel/Redux/React/ESLint comes out).

Also, if you want to share anything fancier than plain data between a worker and the main thread you’ve got some more reading to do (comlink is your friend).

What I’m getting at is this: if the benefit is real, but minimal, then you’ve gotta ask if there’s something else you could spend a day or two on with a greater benefit to your users.

This thinking is true of everything, of course, but I’ve found that Web Workers have a particularly poor benefit-to-effort ratio.

Hey David, why you hate Web Workers so bad?

Good question.

This is a doweling jig:

I own a doweling jig. I love my doweling jig. If I need to drill a hole into the end of a piece of wood and ensure that it’s perfectly perpendicular to the surface, I use my doweling jig.

But I don’t use it to eat breakfast. For that I use a spoon.

Four years ago I was working on some fancy animations. They looked slick on a fast device, but janky on a slow one. So I wrote fireball-js, which executes a rudimentary performance benchmark on the user’s device and returns a score, allowing me to run my animations only on devices that would render them smoothly.

Where’s the best spot to run some CPU intensive code that the user didn’t request? On a different thread, of course. A Web Worker was the correct tool for the job.

Fast forward to 2019 and you’ll find me writing a routing algorithm for a mapping application. This requires parsing a big fat GeoJSON map into a collection of nodes and edges, to be used when a user asks for directions. The processing isn’t in response to a user request and the user isn’t waiting on it. And so, a Web Worker is the correct tool for the job.

It was only when doing this that it dawned on me: in the intervening quartet of years, I have seen exactly zero other instances where Web Workers would have improved the user experience.

Contrast this with a recent resurgence in Web Worker wonderment, and combine that contrast with the fact that I couldn’t think of anything else to write about, then concatenate that combined contrast with my contrarian character and you’ve got yourself a blog post telling you that maybe Web Workers are a teeny-tiny bit overhyped.

Thanks for reading

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Further reading

An Introduction to Web Workers

JavaScript Web Workers: A Beginner’s Guide

Using Web Workers to Real-time Processing

How to use Web Workers in Angular app

Using Web Workers with Angular CLI


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