Using `this` with array methods

In this post we explore a little known method of defining `this` within array method callbacks.

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A contrived example

A fairly standard pattern to see - inline arrow functions used for callbacks:

const animals = ['cat', 'dog', 'monkey'];
const cat = animals.find((animal) => animal === 'cat');
const monkey = animals.find((animal) => animal === 'monkey');

Oh wait, we’ve got some duplication in those arrow functions. You might be tempted to write:

// Utility
const findAnimalInList = (list, animal) => {
	return list.find((item) => item === animal);
};

const animals = ['cat', 'dog', 'monkey'];
const cat = findAnimalInList(animals, 'cat');
const monkey = findAnimalInList(animals, 'monkey');

Great, duplication removed and we can extract that findAnimalInList function into a new file and test it, nice.

But, findAnimalInList is still recreating the callback function provided to find every time it’s invoked. Also, we are abstracting more than we need. If you came to this code with fresh eyes, to know what it is actually doing, we’d need to find the extracted function, see what it does and jump back to where we were.

We’ve lost the context.

There is another way

this way (every pun intended) allows us to extract the logic without loosing all the context.

Ever noticed in the docs that Array.find, Array.some, Array.every, Array.filter etc all accept a weird thisArg argument (usually as the last optional argument)?

This allows us to define this inside of the callback function. If we focus on extracting the logic only, not the find, we can retain some of the context.

// Utility
function byName(animal) {
	return animal === this;
}

const animals = ['cat', 'dog', 'monkey'];
const cat = animals.find(byName, 'cat');
const monkey = animals.find(byName, 'monkey');

You’ll see we’re using a good old fashioned function, the reason being that arrow functions do not have their own this.

Like the previous example we can still isolate+test and it keeps the code DRY, without creating multiple functions. We also have the added bonus in that we could pass the byName function to any array function!

const justCats = animals.filter(byName, 'cat');

This example is using strings, but this could also be an object or an array.

Think of all the possibilities!