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Array Prototype Methods

3 mins 📖 📖 

Array.prototype methods are super useful for transforming array data and cover a lot of common needs in modern applications! They are a form of higher-order function (HOC), which are functions that take another function as their argument, or return one as a result. A good way to view functions is as values, because then it's easier to imagine being able to pass them around. For me, they've been a great introduction into the world of functional programming. In functional programming, the functions you create are pure, i.e. they have no side effects, rely on the input you give them, and will always produce the same output given the same input.

So, I use array methods more and more at work and they are definitely a pleasure to work with! There are quite a lot of them (I think around 50), but there are some more frequently used ones that cover most use cases, such as sort(), map(), filter(), and reduce(). I was inspired to write about them after reading about functional JavaScript post by Angelos Chalaris! Please check out his post because there are great examples & explanations in there!

Of course, since I like them so much, I wanted to give an example here. The one I used most at work is map() but I didn't choose it because I gave an example of it in Day 2 :) As a side note, if you look carefully at the example in Day 2's lesson, you'll see how the elements array takes the map function, which in turn takes a function as its input, and returns a value based on the input. Below, I decided to create an example of reduce().

const initialCatRating = [10, 10, 10]
const accurateCatRating = initialCatRating.reduce((total, amount) => {
total.push(amount * 2)
return total
}, [])

accurateCatRating // [20, 20, 20]

This example shows one of the many things reduce can do - alter values in an array, and push the altered values into a new array! This means a brand new array is returned, rather than altering the initial array. Let me explain what's happening:

There is an initialCatRating variable, which shows ratings of cats out of ten. In the accurateCatRating variable, reduce() is called on initialCatRating.

The first argument in reduce() is a callback function (see this in pink above). The callback takes at least two and up to four arguments (accumulator, currentValue, currentIndex, array). The above example has taken two - total and amount, which are actually accumulator and currentValue. Naming doesn't matter here, but order does.

The second argument in reduce() is initialValue, which is optional. In the above example, initialValue is an empty array. The callback only operates on every element in an array when initialValue is present, so for the above example it is not optional.

Putting the callback and initialValue together allows reduce() to take each element in the given array and operate on it, then push the returned values (using push() - another array method) into a new array.

Of course, to make it re-usable, you can extract the callback function from the above example into something like:

const doubleMyValue = (total, amount) => {
total.push(amount * 2)
return total

Then, it can be put straight into reduce() (as an argument), to read:

const initialCatRating = [10, 10, 10]
const accurateCatRating = initialCatRating.reduce(doubleMyValue, [])

accurateCatRating // [20, 20, 20]

The reduce() callback above is actually not a pure function, because it modifies one of its parameters. To keep reduce() pure/functional, the callback can only take primitive values. See the MDN Array.reduce article for examples. And see some more examples on Array.reduce. Try some Array.prototype methods out in your browser console! :)