Zachary Palmer

Zachary Palmer


Function Composition in JavaScript with Array.prototype.reduceRight

Functional programming in JavaScript has rocketed in popularity over the last few years. While a handful of its regularly-promoted tenets, such as immutability, require runtime workarounds, the language’s first-class treatment of functions has proven its support of composable code driven by this fundamental primitive. Before covering how one can dynamically compose functions from other functions, let’s take a brief step back.

What is a Function?

Effectively, a function is a procedure that allows one to perform a set of imperative steps to either perform side effects or to return a value. For example:

function getFullName(person) {
  return `${person.firstName} ${person.surname}`;

When this function is invoked with an object possessing firstName and lastName properties, getFullName will return a string containing the two corresponding values:

const character = {
  firstName: 'Homer',
  surname: 'Simpson',

const fullName = getFullName(character);

console.log(fullName); // => ‘Homer Simpson’

It’s worth noting that, as of ES2015, JavaScript now supports arrow function syntax:

const getFullName = (person) => {
return ${person.firstName} ${person.surname};

Given our getFullName function has an arity of one (i.e. a single argument) and a single return statement, we can streamline this expression:

const getFullName = person => ${person.firstName} ${person.surname};

These three expressions, despite differing in means, all reach the same end in:

  • creating a function with a name, accessible via the name property, of getFullName
  • accepting a sole parameter, person
  • returning a computed string of person.firstName and person.lastName, both being separated by a space

Combining Functions via Return Values

As well as assigning function return values to declarations (e.g. const person = getPerson();), we can use them to populate the parameters of other functions, or, generally speaking, to provide values wherever JavaScript permits them. Say we have respective functions which perform logging and sessionStorage side effects:

const log = arg => {
return arg;

const store = arg => {
sessionStorage.setItem(‘state’, JSON.stringify(arg));
return arg;

const getPerson = id => id === ‘homer’
? ({ firstName: ‘Homer’, surname: ‘Simpson’ })
: {};

We can carry out these operations upon getPerson‘s return value with nested calls:

const person = store(log(getPerson(‘homer’)));
// person.firstName === ‘Homer’ && person.surname === ‘Simpson’; => true

Given the necessity of providing the required parameters to functions as they are called, the innermost functions will be invoked first. Thus, in the above example, getPerson‘s return value will be passed to log, and log‘s return value is forwarded to store. Building statements from combined function calls enables us to ultimately build complex algorithms from atomic building blocks, but nesting these invocations can become unwieldy; if we wanted to combine 10 functions, how would that look?

const f = x => g(h(i(j(k(l(m(n(o(p(x))))))))));

Fortunately, there’s an elegant, generic implementation we can use: reducing an array of functions into a higher-order function.

Accumulating Arrays with Array.prototype.reduce

The Array prototype’s reduce method takes an array instance and accumulates it into a single value. If we wish to total up an array of numbers, one could follow this approach:

const sum = numbers =>
numbers.reduce((total, number) => total + number, 0);

sum([2, 3, 5, 7, 9]); // => 26

In this snippet, numbers.reduce takes two arguments: the callback which will be invoked upon each iteration, and the initial value which is passed to said callback’s total argument; the value returned from the callback will be passed to total on the next iteration. To break this down further by studying the above call to sum:

  • our callback will run 5 times
  • since we are providing an initial value, total will be 0 on the first call
  • the first call will return 0 + 2, resulting in total resolving to 2 on the second call
  • the result returned by this subsequent call, 2 + 3, will be provided to the total parameter on the third call etc.

While the callback accepts two additional arguments which respectively represent the current index and the array instance upon which Array.prototype.reduce was called, the leading two are the most critical, and are typically referred to as:

  • accumulator – the value returned from the callback upon the previous iteration. On the first iteration, this will resolve to the initial value or the first item in the array if one is not specified
  • currentValue – the current iteration’s array value; as it’s linear, this will progress from array[0] to array[array.length - 1] throughout the invocation of Array.prototype.reduce

Composing Functions with Array.prototype.reduce

Now that we understand how to reduce arrays into a single value, we can use this approach to combine existing functions into new functions:

const compose = (…funcs) =>
initialArg => funcs.reduce((acc, func) => func(acc), initialArg);

Note that we’re using the rest params syntax () to coerce any number of arguments into an array, freeing the consumer from explicitly creating a new array instance for each call site. compose also returns another function, rendering compose a higher-order function, which accepts an initial value (initialArg). This is critical as we can consequently compose new, reusable functions without invoking them until necessary; this is known as lazy evaluation.

How do we therefore compose other functions into a single higher-order function?

const compose = (…funcs) =>
initialArg => funcs.reduce((acc, func) => func(acc), initialArg);

const log = arg => {
return arg;

const store = key => arg => {
sessionStorage.setItem(key, JSON.stringify(arg));
return arg;

const getPerson = id => id === ‘homer’
? ({ firstName: ‘Homer’, surname: ‘Simpson’ })
: {};

const getPersonWithSideEffects = compose(

const person = getPersonWithSideEffects(‘homer’);

In this code:

  • the person declaration will resolve to { firstName: ‘Homer’, surname: ‘Simpson’ }
  • the above representation of person will be output to the browser’s console
  • person will be serialised as JSON before being written to session storage under the person key

The Importance of Invocation Order

The ability to compose any number of functions with a composable utility keeps our code cleaner and better abstracted. However, there is an important point we can highlight by revisiting inline calls:

const g = x => x + 2;
const h = x => x / 2;
const i = x => x ** 2;

const fNested = x => g(h(i(x)));

One may find it natural to replicate this with our compose function:

const fComposed = compose(g, h, i);

In this case, why does fNested(4) === fComposed(4) resolve to false? You may remember my highlighting how inner calls are interpreted first, thus compose(g, h, i) is actually the equivalent of x => i(h(g(x))), thus fNested returns 10 while fComposed returns 9. We could simply reverse the invocation order of the nested or composed variant of f, but given that compose is designed to mirror the specificity of nested calls, we need a way of reducing the functions in right-to-left order; JavaScript fortunately provides this with Array.prototype.reduceRight:

const compose = (…funcs) =>
initialArg => funcs.reduceRight((acc, func) => func(acc), initialArg);

With this implementation, fNested(4) and fComposed(4) both resolve to 10. However, our getPersonWithSideEffects function is now incorrectly defined; although we can reverse the order of the inner functions, there are cases where reading left to right can facilitate the mental parsing of procedural steps. It turns out our previous approach is already fairly common, but is typically known as piping:

const pipe = (…funcs) =>
initialArg => funcs.reduce((acc, func) => func(acc), initialArg);

const getPersonWithSideEffects = pipe(

By using our pipe function, we will maintain the left-to-right ordering required by getPersonWithSideEffects. Piping has become a staple of RxJS for the reasons outlined; it’s arguably more intuitive to think of data flows within composed streams being manipulated by operators in this order.

Function Composition as an Alternative to Inheritance

We’ve already seen in the prior examples how one can infinitely combine functions into to larger, reusable, goal-orientated units. An additional benefit of function composition is to free oneself from the rigidity of inheritance graphs. Say we wish to reuse logging and storage behaviours based upon a hierarchy of classes; one may express this as follows:

class Storable {
constructor(key) {
this.key = key;

store() {
JSON.stringify({ …this, key: undefined }),

class Loggable extends Storable {
log() {

class Person extends Loggable {
constructor(firstName, lastName) {
this.firstName = firstName;
this.lastName = lastName;

debug() {

The immediate issue with this code, besides its verbosity, is that we are abusing inheritance to achieve reuse; if another class extends Loggable, it is also inherently a subclass of Storable, even if we don’t require this logic. A potentially more catastrophic problem lies in naming collisions:

class State extends Storable {
store() {
return fetch(‘/api/store’, {
method: ‘POST’,

class MyState extends State {}

If we were to instantiate MyState and invoke its store method, we wouldn’t be invoking Storable‘s store method unless we add a call to within, but this would then create a tight, brittle coupling between State and Storable. This can be mitigated with entity systems or the strategy pattern, as I have covered elsewhere, but despite inheritance’s strength of expressing a system’s wider taxonomy, function composition provides a flat, succinct means of sharing code which has no dependence upon method names.


JavaScript’s handling of functions as values, as well as the expressions which produce them, lends itself to the trivial composition of much larger, context-specific pieces of work. Treating this task as the accumulation of arrays of functions culls the need for imperative, nested calls, and the use of higher-order functions results in the separation of their definition and invocation. Additionally, we can free ourselves of the rigid hierarchical constraints imposed by object-orientated programming.

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Abigale  Yundt

Abigale Yundt


Forms of Composition in JavaScript and React

One of the core ideas in functional programming is composition: building larger things from smaller things. The canonical example of this idea should be familiar with legos.

Multiple legos can be joined and yield another lego that can continue to be built on and attached to others. In functional programming, the basic unit for composition is functions and larger functions are built by connecting and combining smaller ones.

When asked how to handle a particular scenario, edge case, or requirement, a functional programmer will nearly always answer: ‘with a function’.

Object-oriented concepts like factories, strategy mangers, or polymorphism don’t have an equivalent in the functional paradigm. Functional programming has its own key concepts, composition is one.

A (quick) Aside

One distinction between functional and object oriented programming is best seen in the difference between

circle.area()andarea(circle). In the first version - a more object oriented style -areais a method that exists on theCircleclass and can only be called onCircleinstances. In the second,areais a function that accepts an object. Unlike the object oriented version, it can act on any object that conforms to the type expected byarea.

This illustrates a core difference between the two paradigms. In an object oriented world, data and functionality are coupled -

areais a function on instances ofCircleobjects limited to objects created by that class. In functional programming, data and functionality are decoupled -areais a function that can act on any object that has the required properties.

While object oriented patterns have interfaces, inheritance and other mechanisms for sharing behavior like calculating the area of various shapes, it can often feel like you’re standing in an abandoned abstract factory churning out reverse proxies* 🙃

*I’ve never written an abstract factory and this is just a cheeky line to maintain engagement. Like all things, OO is another tool to leverage when needed.

Forms of Composition

Functions are not the sole unit of composition and the principle extends beyond the domain of functional programming. ReactJS uses components as a unit of composition. Hooks too like

useStateare another unit. If you’re really focusing, you may notice that hooks are just regular functions which is why they are great for composition.

Its possible to build larger components from smaller components, and write custom hooks that extend the capability of existing hooks.

Composition relies on recursion to build larger abstractions from smaller abstractions but with each abstraction layer as the same type as all the others. Once a compositional unit like functions or components exist, you immediately get a compositional model that allows for building high level abstractions very quickly for free. Each layer of abstraction is fundamentally the same type of thing as all the other layers.

An Example of (Functional) Composition

Let’s begin with three functions.

const toUpperCase = str => str.toUpperCase();
const appendExclamationPoints = str => str + '!';
const split = str => str.split('');

Often code takes the output of one function and uses it as the input to another. This is the idea of a pipeline. Data in, data out.

split(appendExclamationPoints(toUpperCase('hello world'))) 
// ["HELLO", "WORLD!"]

While the above works, it isn’t the easiest to read. A simpler abstraction is a single function that can be invoked with some string passed as the parameter.

function appendExclamationPointAndSplitOnSpace(someString) {
    return (someString.toUpperCase() + '!').split();

appendExclamationPointAndSplitOnSpaceagic('hello world') 
// ["Hello", "World!"]

The above function, while meeting the requirements perfectly, isn’t necessarily clean code. It uses an imperative style, specifying each operation to perform to get the desired result. While it may be simple enough to read and understand, a more declarative style would be even easier.

Functional programming can help simplify the above through a helper function called

compose. Compose accepts an arbitrary number of functions, and returns a new function that runs each of the functions passed in such that the output of the previous functions is used as the input to the next

t appendExclamationPointAndSplitOnSpace = compose(

appendExclamationPointAndSplitOnSpace('hello world') 
// ["Hello", "World!"]

Note that the functions execute in a right to left wrapping manner similar to the original example. That is,

splitinvokes the result ofappendExclamationPointswhich invokes the result oftoUpperCase. This results in a declarative style, with no direct function calls or references to the data and methods that exist on the data type. A new function is created that accepts the data and computes the result. Most importantly, we’re able to build the new function from existing smaller functions that we already have or are trivial to write.

Composing functions requires adherence to the following intuitive rule. The output type of function A must match the input type of function B given that B runs with the output from function A. In a practical example, if a function that returns a number is composed with a function that expects a string, some unexpected errors might creep in.

Various implementations of

composecan be found in Lodash, Redux, and other JavaScript libraries. Its certainly not magical, and can be written in under 10 lines.

#functional-programming #javascript #reactjs #composition #pure-functions #compositions-in-javascript #compositions-in-react #javascript-top-story

Vincent Lab

Vincent Lab


The Difference Between Regular Functions and Arrow Functions in JavaScript

Other then the syntactical differences. The main difference is the way the this keyword behaves? In an arrow function, the this keyword remains the same throughout the life-cycle of the function and is always bound to the value of this in the closest non-arrow parent function. Arrow functions can never be constructor functions so they can never be invoked with the new keyword. And they can never have duplicate named parameters like a regular function not using strict mode.

Here are a few code examples to show you some of the differences = "Bob";

const person = {
name: “Jon”,

<span style="color: #008000">// Regular function</span>
func1: <span style="color: #0000ff">function</span> () {
    console.log(<span style="color: #0000ff">this</span>);

<span style="color: #008000">// Arrow function</span>
func2: () =&gt; {
    console.log(<span style="color: #0000ff">this</span>);


person.func1(); // Call the Regular function
// Output: {name:“Jon”, func1:[Function: func1], func2:[Function: func2]}

person.func2(); // Call the Arrow function
// Output: {name:“Bob”}

The new keyword with an arrow function
const person = (name) => console.log("Your name is " + name);
const bob = new person("Bob");
// Uncaught TypeError: person is not a constructor

If you want to see a visual presentation on the differences, then you can see the video below:

#arrow functions #javascript #regular functions #arrow functions vs normal functions #difference between functions and arrow functions

Javascript Array From Example | Array.prototype.from()

Javascript array from() is an inbuilt function that creates a new, shallow-copied array instance from an array-like object or iterable object.

The Array .from() lets you develop Arrays from the array-like objects (objects with a length property and indexed items) or  iterable objects ( objects where you can get its items, such as Map and  Set).

The Array from() function was introduced in ECMAScript 2015.

Javascript Array From Example

Array.from() method in Javascript is used to creates a new  array instance from a given array. If you pass a  string to the Array.from() function, then, in that case, every alphabet of the string is converted to an element of the new array instance. In the case of integer values, a new array instance simply takes the elements of the given Array.

The syntax of the Array.from() method is the following.


Array.from(arrayLike[, mapFn[, thisArg]])

#javascript #ecmascript #javascript array from #array.prototype.from

Lowa Alice

Lowa Alice


JavaScript Arrays Tutorial. DO NOT MISS!!!

Learn JavaScript Arrays

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#arrays #javascript #javascript arrays #javascript arrays tutorial

Javascript Array Shift Example | Array.prototype.shift()

Javascript array shift() is an inbuilt function that removes the first item from an array and returns that deleted item. The shift() method changes the length of the array on which we are calling the shift() method. Javascript Array Shift method is not  pure function as it directly modifies the  array.

Javascript Array Shift Example

Javascript shift() method removes the item at the zeroeth index and shifts the values at consecutive indexes down, then returns that removed value.

If we want to remove the last item of an array, use the  Javascript pop() method.

If the length property is 0,  undefined is returned.

The syntax for shift() method is the following.


An array element can be a  string, a number, an  array, a boolean, or any other  object types that are allowed in the Javascript array. Let us take a simple example.

#javascript #array.prototype.shift #javascript pop #javascript shift