---
title: "Why is this so hard?"
url: "https://www.bekk.christmas/post/2018/03/why-is-this-so-hard"
authors: ["Svein Petter Gjøby"]
published: "2018-12-03"
updated: "2024-11-07T13:05:28Z"
language: "en-US"
type: "article"
tags: ["React"]
---

# Why is this so hard?

I've lost count of the number of times I forgot to bind this. Why do we need to bind this and how should we do it?

## In short

In JavaScript, `this` is set when a function is called, not when it is defined, so function calls can lose their intended context unless they are bound explicitly. In React, event handlers often lose their implicit binding, which is why `this` becomes `undefined` unless you bind the method, use an arrow function, or use public class field syntax.

- **For:** React developers working with class components
- **Good to know beforehand:** Basic JavaScript functions and React class components

_AI-generated_

# Why is this so hard?

To better understand why we need to bind `this` in React, we should understand how `this`[works in JavaScript](https://github.com/getify/You-Dont-Know-JS/blob/master/this%20%26%20object%20prototypes/ch2.md). Let's take a shot at it with the following snippet.

```javascript
class Santa() {
  constructor() {
    this.catchphrase = "Ho ho ho ho!";
  }

  greet() {
    return this.catchphrase;
  }
}

const santa = new Santa();
console.log(santa.greet()); // Ho ho ho ho!
```

Awesome! That worked as expected, but will this?

```javascript
const santa = new Santa();
const greet = santa.greet;
console.log(greet()); // Uncaught TypeError: Cannot read property 'catchphrase' of undefined
```

Wait, what!? We got a `TypeError` saying `catchphrase` isn't a property of `undefined`. Why is that?

In JavaScript, `this` is determined when a function is called, not when the function is defined. Moreover, `this` refers to the context of the function.

In the above example we create a new variable `greet`. `greet` is a top level variable so its context is `undefined`, and `undefined` does not have a property named `catchphrase`. Unlike `santa.greet` which has `santa` as its context, due to implicit binding.

To better understand implicit binding, consider this example.

```javascript
const santa = new Santa();
const greet = santa.greet;

const sheldon {
  catchphrase: "Bazinga!"
}

sheldon.greet = greet;
console.log(sheldon.greet()); // Bazinga!
```

Bazinga! This works because the `greet` function is called in the context of `sheldon`, which has a property named `catchphrase`.

## Binding it ourselves

Sometimes we might want to specify the context of a function. This can be achieved with the `bind` function, and is called explicit binding.

```javascript
const santa = new Santa();
const badGreet = santa.greet;

const goodGreet = badGreet.bind(santa);
console.log(goodGreet()); // Ho ho ho ho!
console.log(badGreet()); // Uncaught TypeError: Cannot read property 'catchphrase' of undefined
```

Calling `bind` on a function returns a copy of the funciton with the argument as the context. In other words `this` is always whatever argument you pass to `bind` for that function. This even works if the implicit context is changed.

```javascript
sheldon.greet = goodGreet;
console.log(sheldon.greet()); // Ho ho ho ho!
```

## What about React?

Let's transform our example into a React component.

```jsx
class Santa extends React.Component {
  constructor(props) {
    super(props);

    this.state = {
      catchphrase: "Ho ho ho ho!",
    };
  }

  render() {
    return <button onClick={this.sayCatchphrase}>Say catchphrase</button>;
  }

  sayCatchphrase() {
    console.log(this.state.catchphrase);
  }
}
```

Clicking the button should ideally print `Ho ho ho ho!` to the console. However, it does **not** in the above example.

In the `sayCatchphrase` function `this` would be `undefined`. This is because the event handler function loses its implicitl binding. As mentioned previously, the context is determined when the function is called. When the event occurs and the function is invoked, the context falls back to the default binding and is set to `undefined`.

To fix this we have a couple of options. We could bind the function ourselves:

```jsx
<button onClick={this.sayCatchphrase.bind(this)}>Say catchphrase</button>
```

> It is considered best practice to [avoid binding your functions in the render method.](https://medium.freecodecamp.org/why-arrow-functions-and-bind-in-reacts-render-are-problematic-f1c08b060e36)

We can use an [arrow function](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions). This works because `this` is bound [lexically](https://github.com/getify/You-Dont-Know-JS/blob/master/this%20%26%20object%20prototypes/ch2.md#lexical-this). This means that it uses the context of the `render` method as its context.

```jsx
<button onClick={() => this.sayCatchphrase()}>Say catchphrase</button>
```

Or we can use the [public class field syntax](https://babeljs.io/docs/en/babel-plugin-proposal-class-properties), which is still experimental and a babel plugin is required to use it.

```jsx
class Santa extends React.Component {
  constructor(props) {
    super(props);

    this.state = {
      catchphrase: "Ho ho ho ho!",
    };
  }

  render() {
    return <button onClick={this.sayCatchphrase}>Say catchphrase</button>;
  }

  sayCatchphrase = () => {
    console.log(this.state.catchphrase);
  };
}
```

That's it!

## Relevant resources recommended by the author

- [Understanding the this keyword](https://medium.com/quick-code/understanding-the-this-keyword-in-javascript-cb76d4c7c5e8)
- [A book about this](https://www.oreilly.com/library/view/you-dont-know/9781491905142/)
- [The MDN docs](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/this)

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