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Functional Programming in JavaScript

What Is Functional Programming?

Functional programming is a programming paradigm designed to handle pure mathematical functions. This paradigm is totally focused on writing more compounded and pure functions.

Functional programming is the process of building software by composing pure functions , avoiding shared state, mutable data, and side-effects.

Pure Functions

As a software programmer/developer, you write source code to produce an output based on the inputs. Usually, you write functions to perform the tasks based on inputs and produce an output. We need to make sure these functions are,

Predictable: It produces a predictable output for the same inputs.

  • [] Readable: Anyone reading the function as a standalone unit can understand its purpose completely.
  • [] Reusable: Can reuse the function at multiple places of the source code without altering its and the caller’s behaviour.
  • [] Testable: We can test it as an independent unit.

A Pure Function has all the above characteristics. It is a function that produces the same output for the same input. It means it returns the same result when you pass the same arguments. A pure function shouldn’t have any side effects to change the expected output.

Example 1 :

function sayGreeting(name){
return `Hello ${name}`;

Example 2:

let greeting = "Hello";
function notPureGreeting(name){
return `${greeting} ${name}`;

The function sayGreeting() is a pure function.

Can you guess why?

It is a pure function because you always get a Hello <name> as output for the <name> pass as an input.

Function notPureGreeting is not a pure function because the function’s output now depends on an outer state called greeting.

What if someone changes the value of the greeting variable to Hola?

It will change the output of the notPureGreeting() function even when you pass the same input.

One of the reasons that pure functions are so powerful is that they are way easier to test and debug. Since a pure function should return the same output given specific input value, we can test this function very easily.

First-Class Objects

Functions in JavaScript are first-class objects, also known as “first-class citizens”. This means that we can work with functions in JavaScript the same way as variables.

In fact, JavaScript functions are themselves types of objects. A first-class function can thus be expected to support the same operations we would expect from other objects. So what are these operations?

Generally, first-class objects can:

  • Be stored in a variable;
  • Be passed as arguments to functions;
  • Be returned by functions;
  • Be stored in some data structure; and,
  • Hold their own properties and methods.
  • Storage in a Variable
// Function definition and invocation
function speak(string) {
speak("Hello");                     // logs "Hello"

// Store in a variable
var talk = speak;
talk("Hi");                         // logs "Hi"

We declare our function speak() and subsequently demonstrate its invocation. So far things are looking pretty standard. Then, we define a variable talkand assign in the value of speak. When we call talkas a function we get the same functionality as when calling speak! This shows that you can store a function in a variable. Not only that, but in this case both speakand talkrefer to the same function, and the expression speak === talk will return true

Passage as an Argument to a Function; Return Value from a Function

// Pass as an argument to a function
// Return from a function
function functionReturner(fn) {
  return fn;
var chat = functionReturner(talk);
chat("Good Morning");               // logs "Good Morning"

We declare another function, self-consciously called functionReturner, which serves the somewhat silly purpose of receiving a function as an argument and simply returning it. And indeed, when we pass functionReturnerour function talkas an argument and assign its return value to the variable chat, we get exactly the same functionality as we did with talk or speak. This is a very arbitrary example, but it shows that functions can indeed be passed as arguments to other functions, and that they can be the return value from other functions too— just like any other first-class object.

Storage in a Data Structure

// Function definition and invocation
function speak(string) {
speak("Hello");                     // logs "Hello"

// Store in a variable
var talk = speak;
talk("Hi");                         // logs "Hi"

// Store in a data structure
var myFuncs = [talk];
myFuncs[0]("Good Afternoon");

Functions can also be stored in data structures. In this example, we store our function talkinside an array called myFuncs. When we attempt to access myFuncs[0], which we expect to be a function, and call it, we do in fact get the functionality we are expecting. A function can similarly be stored on other data structures, such as an object, in which case we generally refer to it as a method of that object.

Owner of a Property

// Function definition and invocation
function speak(string) {
speak("Hello");                     // logs "Hello"

// Store in a variable
var talk = speak;
talk("Hi");                         // logs "Hi"
// Owns properties
talk.myProperty = "bananas";
console.log(talk.myProperty);       // logs "bananas"

We assign our function talk an arbitrary property called myPropertyand store the value “bananas”inside it. Just like with any other object, we can directly access this property.

Uses of a First-Class Function

So, we know that functions in JavaScript are first-class citizens. Now what do we do with that knowledge? . First-class functions give us a wide variety of flexible and powerful design patterns. These patterns let us write more readable, more dynamic, and more concise code. Let’s take a look at a few.

Higher Order Function

A higher-order function is one that either has a function as a parameter, or returns a function.

In other words, higher-order functions do work on other functions. The classic example of higher-order functions are built-in functions that we use every day to manipulate JavaScript objects and data structures. These built-in functions do some kind of work such as iteration or transformation. Often, we supply such built-in functions with anonymous functions as callback arguments, but we can also pass in existing functions.

Consider the following:

var myNums = [1, 2, 3, 4, 5];

function doubleNum(num) {
  return num * 2;

// Built-in Array.prototype.map function, using anonymous function argument
var doubledNums = myNums.map(function(num) {
  return num * 2;
console.log(doubledNums);           // logs "[2, 4, 6, 8, 10]"

// Built-in Array.prototype.map function, using named callback argument
var otherDoubledNums = myNums.map(doubleNum);
console.log(otherDoubledNums);      // logs "[2, 4, 6, 8, 10]"

In this snippet, we first define a variable myNums which contains an array of numbers. We also define a function doubleNum which accepts a number as an argument and returns double the value of that number. We then demonstrate higher-order functions using built-in function Array.prototype.map, which iterates over an array, transforming each value and returning a new array with those values. In the first example, we provide map with an anonymous function that doubles each value in the array. We pass map our predefined function doubleNum, which it then uses in the transformation process. In both cases we get the same result.

Asynchronous Function

Another common use of first-class function is when an asynchronous callback is necessary. This is often the case in web applications where some function must wait to be called until a server has returned some value or a promise has been fulfilled. We won’t get into that here but we can simulate delaying a function call using the built-in function setTimeout, which accepts a callback and a delay as values. Here is an example using our trusty speak function.

function speak(string) {

var delayedFunction = function(fn) {
  return function(val, delay) {
    setTimeout(function() {
    }, delay);

var delayedSpeak = delayedFunction(speak);
delayedSpeak("I'm late!", 1000);      // logs "I'm late" after a 1 second delay

We define a new higher-order function called delayedFunction, which accepts a function as a an argument and then returns a new function, which accepts a value and a delay. The new function uses setTimeoutto wait delay length before executing the original function we provided to delayedFunctionwith the provided value as its argument. We then use delayedFunctionpassing in our speakfunction, to create a new function called delayedSpeak. Now we can use speakwith some delay! Here we have not only used first-class functions, and higher-order functions, but also seen how to asynchronously execute our code.

Function composition

Function composition is an approach where the result of one function is passed on to the next function, which is passed to another until the final function is executed for the final result. Function compositions can be composed of any number of functions.

Traditional approach

Traditionally, function composition follows this format:

const double = x => x * 2
const square = x => x * x
// Tradition approach
var output1 = double(2);
var output2 = square(output1);
// variant two
var output_final = square(double(2));

In the code above, we can see that we need to call the double function followed by the square function to square a term that has been doubled. We can do this by either assigning the values individually in variables and calling functions onto them , or we could use a more direct approach.

Alternate approach

Another approach is to use the compose and pipe functions.

compose function

compose function takes any number of functions and invokes them all one after the other:

const compose = (...fns) => x => fns.reduceRight((y, f) => f(y), x); 
const double = x => x * 2
const square = x => x * x

// function composition
var output_final = compose(square, double)(2);

In the code above, we can see that the compose function is implemented using a general approach , so now it can take any number of functions. The output remains the same as before even with using this implementation.

pipe function On the other hand, we can reverse the order of the function invocation by using the pipe function:

// function composition using pipe of any number of functions
const pipe = (...fns) => x => fns.reduce((y, f) => f(y), x); 
const double = x => x * 2
const square = x => x * x

// function pipe
var output_final = pipe(square, double)(2);

In the code above, we can see that the pipefunction is implemented using a general approach, so it can now take any number of functions. This is similar to the previous composefunction, except that it uses reduceinstead of the reduceRight method. The output is different in this case because the square function is invoked before the double function while, in our compose function, it was in the opposite order.

Immutability in JavaScript

In one translation, it can mean unchanging — however, setting something up as a let or var and then not allowing it to change might as well be as good as using const.

There are a lot of people who take this translation a bit too far in its application, and as a result, misunderstand the concept and depth of immutability.

In this piece, we’ll be decrypting the idea of immutability and how it is applied in JavaScript.

What exactly is immutability?

When it comes to object-oriented and functional programming, the concept of immutability is applied at the object level.

It’s to do with the state and how you’re not allowed to modify after it’s been created. Every language has its own system of state management and before we go any further, we need to understand exactly what a state is.

In object-oriented, a state has two parts to it — the properties and the values. The properties are generally static, meaning that they don’t change. The values, however, are expected to be dynamic and therefore changeable.

So if we were to take a step back and observe the state, it is essentially the ‘shape’ and ‘appearance’ of a particular object at any given point in time.

What this means that when properties are added or removed, the overall shape, and therefore state, changes.

Immutability in JavaScript

In JavaScript, all primitive types are weakly immutable by design. Because the shape of undefined , null, boolean, number, bigInt , String and Symbol is singular and flat. It is a single assignment and when a primitive type is initialised, there is a clear understanding of what it’s going to look like.

It’s not going to shrink, expand, or do funny things if you change the assigned value. It might change your processed output when popped into a function, but that’s a different story. If you need it to be strongly immutable, use const.

In JavaScript, custom objects are highly mutable — unless you explicitly tell it not to be. How do you do this? You can either do it via Object.defineProperty() or Object.freeze() Example :

const updateLocation = (data, newLocation) => {
    return {
      Object.assign({}, data, {
        location: newLocation

By using Object.assign(), we can create a function that does not mutate the object passed to it. This will generate a new object instead by copying the second and third parameters into the empty object passed as the first parameter. Then the new object is returned.

Note: updateLocation() is a pure function. If we pass in the first user object, it returns a new user object with a new value for the location.

Declarative vs. Imperative

Functional programming is declarative rather than imperative.

  • Functional Programming is a declarative programming paradigm, in contrast to imperative programming paradigms.
  • Declarative programming is a paradigm describing WHAT the program does, without explicitly specifying its control flow.
  • Imperative programming is a paradigm describing HOW the program should do something by explicitly specifying each instruction (or statement) step by step, which mutate the program’s state.

This “what vs how” is often used to compare both of these approaches because… Well, it is actually a good way to describe them.

Granted, at the end of the day, everything compiles to instructions for the CPU. So in a way, declarative programming is a layer of abstraction on top of imperative programming. The Imperative Code

Let’s break down the thought process required to figure out what’s going on here.

  • JavaScript isn’t typed, so figuring out the return and argument types is the first challenge.
  • We can surmise from the name of the function and the two return statements that return literal boolean values that the return type is boolean.
  • The function name suggests that the two arguments may be arrays, and the use of needle.length and haystack.indexOf confirms that.
  • The loop iterates the needle array and exits the function returning false whenever the currently indexed value of the needle array is not found in the haystack array.
  • If the loop completes without exiting the function, then we found no mismatches and true is returned.
  • Thus, if all the values of the needle array (in any order) are found in the haystack array, we get a true return, otherwise false.

The Declarative Code

That took fewer lines, but you still have to break it down to understand what it’s doing. Let’s see how that process differs.

  • JavaScript isn’t typed, so figuring out the return and argument types is the first challenge.
  • We can surmise from the name of the function and the returned result of an array’s every method that the return type is boolean.
  • The function name suggests that the two arguments may be arrays, as do the default values now added to the arguments for safety.
  • The needle.every call names its current value ‘el’, and checks if it is present in the haystack array using haystack.includes.
  • The needle.every call returns true or false, telling us, quite literally, whether every element in the needle array is included in the haystack array.

Note: Here is the link where you can learn more about Imperative and Declarative

programming: Link

Benefits of Functional Programming

You are writing code to build an application. In development journey, you want to reuse the code of a few lines (100) at different places and for applications functions are helpful. We can write functions at one place and we will be able to access those functions from anywhere in the program. Functional programming has the following features —

  • Reduces code redundancy.
  • Improves modularity.
  • Helps us to solve complex problems.
  • Increases maintainability.
  • Note: Here is the link where you can learn more about the benefits of Functional

Programming: Link

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