1.1.3 • Public • Published

Functional School

A trip through functional programming in Javascript using common built-in Javascript array methods such as map & reduce.

By the end, you should have an understanding of how to use array methods to manipulate semi-complex data.

Level: Intermediate Keywords: javascript, functional Length: 1-2 hours


CodeRoad is an open-sourced interactive tutorial platform for the Atom Editor. Learn more at


  • install the tutorial package

    npm install --save coderoad-functional-school

  • install and run the atom-coderoad plugin



Understanding the Data Set

Over this tutorial series, we'll be changing and working with two different data sets. It'll be a big help to first understand what the data looks like.

var students = [
    "title": "Relational Databases",
    "instructor": "Sean Quentin Lewis",
    "name": "Ada Lovelace",
    "score": 91,
    "grade": "A"

Here we have an array of "student" objects. To get the first item in the array, you can use the array index. Array indexes start at 0.

  'first instructor', students[0].instructor
// first instructor Sean Quentin Lewis

Array -> Array of items that match a condition

You've hacked into the school's computer system, and just in time. The grades are in, but you're not too proud of your performance. That's okay, you have a plan: you're going to create a fake report card.

It would be great if you could filter the scores that your parents will see.

filter takes a matching condition function and only returns items that result in true. As an example, look at isA below:

function isA(x) {
  return x === 'a';

Like all of the methods in this chapter, filter is already part of the Array.prototype, so you can run it following any array. Each item in the array is passed into the params of the condition function, one by one. Learn more.

const list = ['a', 'b'];
// if isA(list[0]), add to output array
// if isA(list[1]), add to output array
//> ['a']

If your data was composed of objects, we could use dot notation to find matches. Checkout isB below.

function isB(x) {
  return x.item === 'b'
const list = [{item: 'a'}, {item: 'b'}];
//> [{item: 'b'}]

Where were we? Back to filtering our grades.

There's too much student data in the computer system. We'll have to sort through it. Have a look at an example below:

//> { course: 'Web Security',
//    instructor: 'Sue Denim',
//    name: 'Rebecca Heineman',
//    score: 93,
//    grade: 'A' }

Array -> sorted Array

Your grades are filtered down to your name and good scores - but wouldn't it be better if your best grades were displayed first, at the top? Besides, your parents rarely read anything through.

You can use the array method sort to arrange your data. Let's see how it works.

['c', 'b', 'a'].sort();
//> ['a', 'b', 'c']
[3, 2, 1].sort();
//> [1, 2, 3]

But what about sorting scores inside of an object?

[{a: 3}, {a: 1}, {a: 2}].sort();
//> [{a: 3}, {a: 1}, {a: 2}]

That didn't work. Instead, you can write a custom compareScore function.

A sort function takes two params, and compares the first to the second. It should return values saying where the second value should go in the array:

  • -1 : sort to a lower index (front)
  • 1 : sort to a higher index (back)
  • 0 : stay the same

Alright, now time to sort your best grades to the top.

First you'll need to write a sort condition function called compareScore.


Array -> run a function over each item -> Array

You've filtered and sorted our data, but neither of those actually change the data.

Wouldn't it be simpler if you could just change your grades?

You can use the array method map to run a function that returns changes to your data.

As an example, let's look at how you would increment each number in an array.

function addOne(num) {
  return num + 1;
[1, 2, 3].map(addOne);
//> [2, 3, 4]
function addToVal(obj) {
  obj.val += 1;
  return obj;
[{ val: 1}].map(addToVal);
//> [{ val: 2 }]

map can change data, and it can also alter structure of the data you're working with.

function makeObject(num) {
  return { val: num };
[1, 2].map(makeObject);
//> [{ val: 1 }, { val: 2 }]

Similarly, map can also restrict the data you want to work with. See the example below to see another way scores could be sorted.

  .map(function(student) {
    return student.score;
//> [93, 91, 88, 88, 82, 81, 73]

In this example, map transformed an object with keys of 'title', 'instructor', 'name', 'score' and 'grade', to an array of just scores. Values weren't changed, but rather limited to a smaller subset of scores.

map is powerful. Let's see what you can do with it.

Those D & F grades would look a lot better if they suddenly became A's.

Let's go back to before we filtered out the bad grades, and instead change the grades to A's.


Array -> run a function for each item

You've updated your grades, but they're still in an array. It's time to loop over them and log them to the console.

To open the console, go to View > Developer > Toggle Developer Tools. Or press cmd+opt+I on Mac, ctrl+alt+I on Windows.

forEach has a lot in common with map, but there is a big difference. Understanding that difference is important for grasping the difference between:

  • functional & imperative programming
  • pure & impure functions

Know it or not, you're probably already used to "imperative" programming.

Imperative programming describes the order of actions

Imperative code tells the computer what to do, step by step.

let x = 1; // make a variable
= x + 1; // add one
= x + 1; // add another
//> 3

Functional programming describes the data transformation

Functional programming is a lot like writing math equations. As in math, 1 + 1 always equals 2.

In the same way, a pure function will always have the same result from a given input. Input 1 -> output 2. Every time.

// a pure function
function addOne(x) {
  return x + 1;
//> 2
//> 2

A function is "pure" if it doesn't change anything outside of its scope. Pure functions are easy to test, reuse and reason about. In other words, they make your job easier.

On the other hand, impure functions are less predictable. The result may be different if you call it at a later time.

let y = 1;
// impure function
function increment(x) {
  y += x;
  return y;
//> 2
//> 3

It's good practice to ensure your map functions remain pure.

But forEach can be a little more dangerous. Why? Let's have a look.

[1, 2, 3].map(addOne);
//> [2, 3, 4]
[1, 2, 3].forEach(addOne);
//> undefined

What? undefined? forEach runs a function on each item in the array, and doesn't care what the function returns. Functions called by forEach must make changes, called side effects, to even be noticed.

// impure function, changes log
function addOneToLog(x) {
[1, 2, 3].forEach(addOneToLog);
//> 2
//> 3
//> 4

Now that we see how forEach works, let's use it to make calls to the console.


Array -> first element that matches a condition

Somehow your name has disappeared from the computer system. We'll have to find a way to get it back.

You quickly put together a list of other students in class. If someone changed your name, it'll be the name that is not in that list.

find works similar to filter, but returns only the first match.

const data = [1, 2, 3, 4, 5, 6];
function isEven(num) {
  return num % 2 === 0;
// returns all matching data to a condition
//> [2, 4, 6]
// returns the first match
//> [2]

Find is great for performantly matching unique values in data, such as an "id", or in our case: a name.


Array + Array -> Array

Before we've been working on a structured set of student data.

// array of students
  "title": "Relational Databases",
  "instructor": "Sean Quentin Lewis",
  "name": "Rebecca Heineman",
  "score": 71,
  "grade": "C"
// students in courses...

To be safe, let's now work on the original data set. Notice how it is structured differently.

// array of courses
    "title": "Relational Databases",
    "instructor": "Sean Quentin Lewis",
    "students": [
        "name": "Rebecca Heineman",
        "score": 71,
        "grade": "C"
    // students...
  // courses...

In this data set, there is an array of students within an array of courses. So how can we recreate our original array of students from the courses?

Weird things happen when you start combining arrays. We can use concat to bring sanity.

[1, 2] + [3, 4];
//> "1, 23, 4"
[1, 2].push([3, 4]);
//> 3
[1, 2].join([3, 4]);
//> "13, 42"
[1, 2].concat([3, 4]);
//> [1, 2, 3, 4]

Unfortunately, Javascript is missing a built in array method to concat multiple arrays together: let's call it flatten (sometimes called concatAll).

flatten should loop over an array and concat each element.

Let's look at an abstraction of what we need to do:

const start = [{
  a: 1,
  c: [
    { b: 1 }
}, {
  a: 2,
  c: [
    { b: 2 }, { b: 3 }
const middle = {
  return {
    return {
      a: outer.a,
      b: inner.b
//> [ [{ a: 1, b: 1 }], [{a: 2, b: 2}, {a: 2, b: 3}] ]
const end = pre.flatten();
//> [{a: 1, b: 1}, {a: 2, b: 2}, {a: 2, b: 3}]

Back to business.

We have a suspect in mind: a classmate named "Hack Kerr". He's a nice guy, and he's always been friendly to you - but there's something suspicious about him: his name.

We'll test out flatten, then re-create our student array of data from the original course data.

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