by Lukas Gisder-Dubé

How to understand the keyword this and context in JavaScript

Photo by beasty . on Unsplash

As mentioned in one of my earlier articles, mastering JavaScript fully can be a lengthy journey. You may have come across this on your journey as a JavaScript Developer. When I started out, I first saw it when using eventListeners and with jQuery. Later on, I had to use it often with React and I am sure you also did. That does not mean that I really understood what it is and how to fully take control of it.

However, it is very useful to master the concept behind it, and when approached with a clear mind, it is not very difficult either.

Digging into this

Explaining this can lead to a lot of confusion, simply by the naming of the keyword.

this is tightly coupled to what context you are in, in your program. Let’s start all the way at the top. In our browser, if you just type this in the console, you will get the window-object, the outermost context for your JavaScript. In Node.js, if we do:

console.log(this)

we end up with {}, an empty object. This is a bit weird, but it seems like Node.js behaves that way. If you do

(function() {
  console.log(this);
})();

however, you will receive the global object, the outermost context. In that context setTimeout , setInterval , are stored. Feel free to play around a little bit with it to see what you can do with it. As from here, there is almost no difference between Node.js and the browser. I will be using window. Just remember that in Node.js it will be the global object, but it does not really make a difference.

Photo by Chor Hung Tsang on Unsplash

Remember: Context only makes sense inside of functions

Imagine you write a program without nesting anything in functions. You would simply write one line after another, without going down specific structures. That means you do not have to keep track of where you are. You are always on the same level.

When you start having functions, you might have different levels of your program and this represents where you are, what object called the function.

Keeping track of the caller object

Let’s have a look at the following example and see how this changes depending on the context:

const coffee = {
  strong: true,
  info: function() {
    console.log(`The coffee is ${this.strong ? '' : 'not '}strong`)
  },
}
coffee.info() // The coffee is strong

Since we call a function that is declared inside the coffee object, our context changes to exactly that object. We can now access all of the properties of that object through this . In our example above, we could also just reference it directly by doing coffee.strong . It gets more interesting, when we do not know what context, what object, we are in or when things simply get a bit more complex. Have a look at the following example:

Classes and Instances

Classes can be used to abstract your code and share behavior. Always repeating the info function declaration in the last example is not good. Since classes and their instances are in fact objects, they behave in the same way. One thing to note however, is that declaring this in the constructor actually is a prediction for the future, when there will be an instance.

Let’s take a look:

class Coffee {
  constructor(strong) {
    this.strong = !!strong
  }
  info() {
    console.log(`This coffee is ${this.strong ? '' : 'not '}strong`)
  }
}
const strongCoffee = new Coffee(true)
const normalCoffee = new Coffee(false)
strongCoffee.info() // This coffee is strong
normalCoffee.info() // This coffee is not strong

Pitfall: seamlessly nested function calls

Sometimes, we end up in a context that we did not really expect. This can happen, when we unknowingly call the function inside another object context. A very common example is when using setTimeout or setInterval :

// BAD EXAMPLE
const coffee = {
  strong: true,
  amount: 120,
  drink: function() {
    setTimeout(function() {
      if (this.amount) this.amount -= 10
    }, 10)
  },
}
coffee.drink()

What do you think coffee.amount is?

...

..

.

It is still 120 . First, we were inside the coffee object, since the drink method is declared inside of it. We just did setTimeout and nothing else. That’s exactly it.

As I explained earlier, the setTimeout method is actually declared in the window object. When calling it, we actually switch context to the window again. That means that our instructions actually tried to change window.amount, but it ended up doing nothing because of the if-statement. To take care of that, we have to bind our functions (see below).

React

Using React, this will hopefully be a thing of the past soon, thanks to Hooks. At the moment, we still have to bind everything (more on that later) in one way or another. When I started out, I had no idea why I was doing it, but at this point, you should already know why it is necessary.

Let’s have a look at two simple React class Components:

When we now click on the button rendered by the Child , we will receive an error. Why? Because React changed our context when calling the _getCoffee method.

I assume that React does call the render method of our Components in another context, through helper classes or similar (even though I would have to dig deeper to find out for sure). Therefore, this.state is undefined and we’re trying to access this.state.coffeeCount . You should receive something like Cannot read property coffeeCount of undefined .

To solve the issue, you have to bind (we’ll get there) the methods in our classes, as soon as we pass them out of the component where they are defined.

How many coffees did you drink so far? / Photo by Ozgu Ozden on Unsplash

Let’s have a look at one more generic example:

We’re passing the increaseCount from one class to another. When we call the increaseCount method in Viking, we have already changed context and this actually points to the Viking , meaning that our increaseCount method will not work as expected.

Solution — bind

The simplest solution for us is to bind the methods that will be passed out of our original object or class. There are different ways where you can bind functions, but the most common one (also in React) is to bind it in the constructor. So we would have to add this line in the Battle constructor before line 18:

this.increaseCount = this.increaseCount.bind(this)

You can bind any function to any context. This does not mean that you always have to bind the function to the context it is declared in (this is the most common case, however). Instead, you could bind it to another context. With bind , you always set the context for a function declaration. This means that all calls for that function will receive the bound context as this . There are two other helpers for setting the context.

Arrow functions `() => {}` automatically bind the function to the declaration context
Photo by Mario Klassen on Unsplash

Apply and call

They both do basically the same thing, just that the syntax is different. For both, you pass the context as first argument. apply takes an array for the other arguments, with call you can just separate other arguments by comma. Now what do they do? Both of these methods set the context for one specific function call. When calling the function without call , the context is set to the default context (or even a bound context). Here is an example:

class Salad {
  constructor(type) {
    this.type = type
  }
}
function showType() {
  console.log(`The context's type is ${this.type}`)
}
const fruitSalad = new Salad('fruit')
const greekSalad = new Salad('greek')
showType.call(fruitSalad) // The context's type is fruit
showType.call(greekSalad) // The context's type is greek
showType() // The context's type is undefined

Can you guess what the context of the last showType() call is?

..

.

You’re right, it is the outermost scope, window . Therefore, type is undefined, there is no window.type

This is it, hopefully you now have a clear understanding on how to use this in JavaScript. Feel free to leave suggestions for the next article in the comments.

About the Author: Lukas Gisder-Dubé co-founded and led a startup as CTO for 1 1/2 years, building the tech team and architecture. After leaving the startup, he taught coding as Lead Instructor at Ironhack and is now building a Startup Agency & Consultancy in Berlin. Check out dube.io to learn more.