Iterator Drill: Reduce

Learning Goals

Define how the reduce() method works
Demonstrate reduce()
Use reduce() to create a single aggregate of all items in a list

Introduction

In the world of programming, we often work with lists. Sometimes we want to transform elements in that list to another value — but other times, we want to aggregate a result. In other words, we want to reduce a list to a single value — it could be, for example, a string, a number, a boolean. For example, our friend has a driver's license and a vehicle, but is very bad at keeping an eye on the parking meter. They've got dozens of parking tickets with fees. Wouldn't it be great if we could reduce all of those parking fees to a single total value?

Define How the `reduce()` Method Works

To illustrate how reduce() works, we'll make up some store data, we're interested in getting a total price of all products in our basket. The store data looks like this:

const products = [
  { name: 'Shampoo', price: 4.99 },
  { name: 'Donuts', price: 7.99 },
  { name: 'Cookies', price: 6.49 },
  { name: 'Bath Gel', price: 13.99 }
];

We're going to reduce the array of products to a single value— in this case the total price of all products in the basket. Let's create a function that has an initial value, then iterates the given products and adds their price to the total price. When the loop has finished, we return our totalPrice result:

function getTotalAmountForProducts(products) {
  let totalPrice = 0;

  products.forEach(function(product) {
    totalPrice += product.price;
  });

  return totalPrice;
}

console.log(getTotalAmountForProducts(products)); // prints 33.46

This is a very basic way to manually add together the prices of the products we want to buy.

To abstract this further, let's count the number of coupons we have lying around the house:

const couponLocations = [
  { room: 'Living room', amount: 5 },
  { room: 'Kitchen', amount: 2 },
  { room: 'Bathroom', amount: 1 },
  { room: 'Master bedroom', amount: 7 }
];

function couponCounter(totalAmount, location) {
  return totalAmount + location.amount;
}

console.log(couponLocations.reduce(couponCounter, 0)); // prints 15

What if we already have three coupons in our hand? We can easily account for that by adjusting the initial value:

console.log(couponLocations.reduce(couponCounter, 3)); // prints 18

Demonstrate `reduce`

With JavaScript’s reduce() method, we don't need to write out all that code. The reduce() method is when we want to get something useful out of each element in the collection and gather that information into a final summary object or value. Let's take the native implementation with our previous example for a spin:

console.log(couponLocations.reduce(couponCounter, 0)); // also prints 15!

Another simple numerical example:

let doubledAndSummed = [1, 2, 3].reduce(function(total, element){ return element * 2 + total}, 0)
// => 12

Here, for each element, JavaScript passes it and the running total (initialized to 0, in the second argument to reduce()) into the function. The function multiplies the element by 2 and adds that to the current total.

That sum (2 * element + total) is the return value of the function and becomes the new total for the next iteration. When there's nothing left to iterate, the total is returned.

The initialization value can be left out but it might lead to a real surprise. If no initial value is supplies, the first element is used without having been used in the function:

let doubledAndSummed = [1, 2, 3].reduce(function(total, element){ return element * 2 + total})
// => 11

The initialization value can be changed:

let doubledAndSummedFromTen = [1, 2, 3].reduce(function(total, element){ return element * 2 + total}, 10)
// => 22

For more powerful uses, we could use:

let hogwarts_houses = {
  "Slytherin": [],
  "Gryffindor": [],
  "Hufflepuff": [],
  "Ravenclaw": []
}

/*

Assume sorting_hat.assign() returns a String ("Slytherin", "Gryffindor",
"Hufflepuff", "Ravenclaw") based on the argument passed in.

*/

incoming_students.reduce(function(houses, student) { houses[sorting_hat.assign(student)].push(student)} , hogwarts_houses)

Here we iterate a collection of students and assign each one to a pre-existing accumulator Object. We ask the Object to look up an Array keyed by the name of the houses. We then push() the student into that Array. Later in the code:

hogwarts_houses["Gryffindor"] //=> [hermioneGranger, ronWeasley, harryPotter]

Use `reduce()` to Create a Single Aggregate of All Items in a List.

Let's say we are hard at work in the battery factory. We've assembled several batches of batteries today. Let's count how many assembled batteries we ended up with.

Create a new variable called totalBatteries which is the sum of all of the battery amounts in the batteryBatches array. Naturally, use reduce() for this!

Conclusion

With reduce(), like many other enumerators in the JavaScript library, we are able to quickly get a minimized list or a total sum from a set of values, including values that we might have filtered out of a list. With this, we can greatly cut down the amount of time spent recreating common functionality, or building out helper methods for commonly used functionality from scratch.

Resources

MDN: Array.prototype.reduce()

Trejon/fewpjs-iterators-fndcl-fnexpr-reduce-lab-online-web-pt-061019