this and arrow functions

August 27, 2018

Arrow functions were introduced in ES6 as a new syntax for writing Javascript functions. Thanks to their short syntax, they encourage the use of small functions, which make code look cleaner (and () => just looks cooler 😄).

As a beginner just getting to wrap my head around ES6 syntax, I began using arrow functions everywhere without really understanding how they worked. As you might expect, I ended up running into some problems, especially with the this keyword.

this can get a bit confusing sometimes as its value varies depending on the function's execution context, and on the mode (strict or non-strict). Much has been written about the ins and outs of that so I'll just focus on one thing:

How this works in arrow functions

In a regular function, this refers to the object when defined as a method of an object. We can therefor do:

const brunch = {
    food: 'Dim sum',
    beverage: 'Jasmine tea',
    order: function() {
        return `I'll have the ${this.food} with ${this.beverage} please.`
    }
}

Calling brunch.order() will return "I'll have the Dim sum with Jasmine tea please."

Let's edit that and use an arrow function for order::

const brunch = {
    food: 'Dim sum',
    beverage: 'Jasmine tea',
    order: () => {
        return `I'll have the ${this.food} with ${this.beverage} please.`
    }
}

This time, calling brunch.order() returns "I'll have the undefined with undefined please." Both this.food and this.beverage return undefined.

It worked with the normal function, so what's going on? In the normal function, this was our order object. When using an arrow function, this is not bound to anything and it just inherits from the parent scope which in this case is the window. Adding a console.log(this) before the return in the arrow function returns a Window object, so its looking for Window.food and Window.beverage which will obviously both be undefined.

Arrow functions are therefor not suited as object methods.

Another common problem area would be with event handlers. Here's an example:

// change button colour on click
<style>
.on {background: red;}
</style>

<button id="click">Toggle</button>

<script>
const button = document.querySelector('#click');
button.addEventListener('click', function() {
    console.log(this); // button
    this.classList.toggle('on');
});
</script>

In the code above, this refers to the button. Clicking on the button toggles the colour as expected. Change the function to an arrow function:

// change button colour on click
<style>
.on {background: red;}
</style>

<button id="click">Toggle</button>

<script>
const button = document.querySelector('#click');
button.addEventListener('click', () => {
    console.log(this); // Window { ... }
    this.classList.toggle('on');
});
</script>

and this becomes the browser's window attribute. Clicking the button will give a TypeError error. If you rely on this in an event hanlder, a regular function may be necessary.

So, as cool and popular as arrow functions may be, its best to understand how they work, and to know when to and not to use them.



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