The Complete Guide To JavaScript's .reduce() Function (With Real-world Examples)

In this guide we are going to cover:

What is the .reduce() function?
Creating our version of the .reduce() function
Calculating the total items and price for a e-commerce shopping cart
Creating an array of unique categories for a restaurant menu
Removing duplicate objects from the array
Replacing .filter().map() with .reduce()
Grouping objects by key (Similar to SQL GROUP BY or C#'s .GroupBy())
How to flatten an array of arrays

Link for all code snippets used in this guide

It seems confusing at first

If you have ever struggled to understand the .reduce() function, you are not alone. It takes messing about with it a couple of times to really understand how it works and use it with confidence.

Most of the examples out there use a simple array of numbers or strings, but the reality is that we normally work with complex objects which is rarely covered.

But before we jump in.
Here's how you can get more out of this content.

Open your code edtitor and try the examples yourself
tinker with it (alter values, add or remove elements to/from the array, etc)
If you find yourself lost in one of the examples, track down the value of the variables either with a pen and paper or on your code editor.

Let's get started!

What is the .reduce() function?

It is a method that executes a callback function on each element of an array.

The return value from the calculation is passed on to the next element.
In the first iteration, there is no "return value of the previous calculation", if passed in, a initial value can be used in it's place.
Otherwise, the first element of the array is used as the initial value and iteration starts from the next element.

To put it simply, it is just a variable and a loop.

The variable holds the new value
The loop iterates over the array so we can grab what we want from each element

This:

Achieves the same as this:

Pretty much every time you need to use a variable and a loop, you can replace it with the reduce function.

As there is no better way of learning than actually trying things out, let's build our version of the .reduce() method so we can really understand it.

Building our own .reduce()

Let's do it by solving a simple problem:
We have an array of numbers and we want the sum of all of them.

It is just like the example above of a variable and a loop.

We need 3 things:

an array
a variable that holds the total value total(usually called the accumulator)
a loop to iterate over the array

Let's say you want to turn it into a function so you can use it easily, how would you do it?

Maybe something along these lines:

We pass the array as a parameter into our function
We return total at the end.

That is looking good, but what if we want the option to set the initial value of the variable total?
We can add the option as another parameter and set total to its value.

Like so:

But there are still 2 problems with this function.

There is no way we can add different logic to be executed inside the loop (It only adds the current element to the sum variable)
It works only on simple arrays (where each item is a value type, not an object)

I'll show you what I mean.

Let's create an array of objects this time

Let's invoke our myReducer function passing in the array of objects

It doesn't work because the value we want to add is inside the object and not the object itself.
We have to access it as total += arr[i].number

But this function didn't account for that.

However, if we add a callback function as a second parameter, we can:

Execute the callback function on each element of the array
Create any custom logic we need

Let's modify our myReducer function.

Let's test it again with the objects array.

First, declare the callback function and call myReduce passing it in.

Note that we added if and else conditions based on the value of total which is our initialValue option.

If initialValue is not given
- it doesn't execute the callback on 1st iteration
- There is nothing to add, initialValue is undefined.
- just set total to the value of the current element
If initialValue is given
- It executes the callback on all iterations

That is pretty much what the original Array.prototype.reduce() does.

The main difference is that there's no need to pass the array as an argument as the original .reduce() is bound to Array.prototype so the array is always the array upon which .reduce() was called.

Calculate total items and price for an ecommerce shopping cart

Say you have an array like this one:

And you need to reduce this array into an object like this { totalItems: 0, totalPrice: 0 } so we can display the correct information on the checkout page.

This is how we can do it:

Create an array of unique categories for a restaurant menu

Given this array:

We need to reduce it into an array of unique categories like ['Appetizer','Entree','Main']

This is how we can do it:

Removing duplicate objects from the array

Similar to the example above, but this time we need to return the same array of objects filtered with only unique objects.

This is the array with a duplicate object:

Note: You have to have a property similar to id that is unique for each object

This is how we can create another array with only unique elements:

Replace .filter().map() with .reduce()

It's very common having to filter an array and then modify the elements of the filtered array.

We will use the same array of the menu categories above

But this time we want to get an array with the itemName only of the items whose category is 'Entree'

We can use .filter() the array and then .map() to accomplish it

Or we can do it with .reduce()

The advantage of using .reduce() is that you iterate over the array only once.

Group objects by key (Similar to SQL GROUP BY or C#'s .GroupBy())

This is the one I like the most. This is a very handy function that will be included in JavaScript's array methods in probably on next update (currently on stage 3 for approval)

This groupBy function can also be used as a middle step to solving more complex problems.

We again will use the menu categories array: