Discover Functional JavaScript was named one of the best new Functional Programming books by BookAuthority!

ES6 brings more features to the JavaScript language. Some new syntax allows you to write code in a more expressive way, some features complete the functional programming toolbox, and some features are questionable.

let and const

There are two ways for declaring a variable (let and const) plus one that has become obsolete (var).


let declares and optionally initializes a variable in the current scope. The current scope can be either a module, a function or a block. The value of a variable that is not initialized is undefined .

Scope defines the lifetime and visibility of a variable. Variables are not visible outside the scope in which they are declared.

Consider the next code that emphasizes let block scope:

let x = 1;
  let x = 2;
console.log(x); //1

In contrast, the var declaration had no block scope:

var x = 1;
  var x = 2;
console.log(x); //2

The for loop statement, with the let declaration, creates a new variable local to the block scope, for each iteration. The next loop creates five closures over five different i variables.

(function run(){
  for(let i=0; i<5; i++){
    setTimeout(function log(){
      console.log(i); //0 1 2 3 4
    }, 100);

Writing the same code with var will create five closures, over the same variable, so all closures will display the last value of i.

The log() function is a closure. For more on closures, take a look at Discover the power of closures in JavaScript.


const declares a variable that cannot be reassigned. It becomes a constant only when the assigned value is immutable.

An immutable value is a value that, once created, cannot be changed. Primitive values are immutable, objects are mutable.

const freezes the variable, Object.freeze() freezes the object.

The initialization of the const variable is mandatory.


Before modules, a variable declared outside any function was a global variable.

With modules, a variable declared outside any function is hidden and not available to other modules unless it is explicitly exported.

Exporting makes a function or object available to other modules. In the next example, I export functions from different modules:

//module "./TodoStore.js"
export default function TodoStore(){}

//module "./UserStore.js"
export default function UserStore(){}

Importing makes a function or object, from other modules, available to the current module.

import TodoStore from "./TodoStore";
import UserStore from "./UserStore";

const todoStore = TodoStore();
const userStore = UserStore();


The operator can be the spread operator or the rest parameter, depending on where it is used. Consider the next example:

const numbers = [1, 2, 3];
const arr = ['a', 'b', 'c', ...numbers];

["a", "b", "c", 1, 2, 3]

This is the spread operator. Now look at the next example:

function process(x,y, ...arr){
//[3, 4, 5]

function processArray(...arr){
//[1, 2, 3, 4, 5]

This is the rest parameter.


With the rest parameter we can replace the arguments pseudo-parameter. The rest parameter is an array, arguments is not.

function addNumber(total, value){
  return total + value;

function sum(...args){
  return args.reduce(addNumber, 0);

sum(1,2,3); //6


The spread operator makes the cloning of objects and arrays simpler and more expressive.

The object spread properties operator will be available as part of ES2018.

const book = { title: "JavaScript: The Good Parts" };

//clone with Object.assign()
const clone = Object.assign({}, book);

//clone with spread operator
const clone = { };

const arr = [1, 2 ,3];

//clone with slice
const cloneArr = arr.slice();

//clone with spread operator
const cloneArr = [ ...arr ];


In the next example, the spread operator is used to concatenate arrays:

const part1 = [1, 2, 3];
const part2 = [4, 5, 6];

const arr = part1.concat(part2);

const arr = [...part1, ...part2];

Merging objects

The spread operator, like Object.assign(), can be used to copy properties from one or more objects to an empty object and combine their properties.

const authorGateway = { 
  getAuthors : function() {},
  editAuthor: function() {}

const bookGateway = { 
  getBooks : function() {},
  editBook: function() {}

//copy with Object.assign()
const gateway = Object.assign({},
//copy with spread operator
const gateway = {

Property short-hands

Consider the next code:

function BookGateway(){
  function getBooks() {}
  function editBook() {}
  return {
    getBooks: getBooks,
    editBook: editBook

With property short-hands, when the property name and the name of the variable used as the value are the same, we can just write the key once.

function BookGateway(){
  function getBooks() {}
  function editBook() {}
  return {

Here is another example:

const todoStore = TodoStore();
const userStore = UserStore();
const stores = {

Destructuring assignment

Consider the next code:

function TodoStore(args){
  const helper = args.helper;
  const dataAccess = args.dataAccess;
  const userStore = args.userStore;

With destructuring assignment syntax, it can be written like this:

function TodoStore(args){
   const { 
      userStore } = args;

or even better, with the destructuring syntax in the parameter list:

function TodoStore({ helper, dataAccess, userStore }){}

Below is the function call:

  helper: {}, 
  dataAccess: {}, 
  userStore: {} 

Default parameters

Functions can have default parameters. Look at the next example:

function log(message, mode = "Info"){
  console.log(mode + ": " + message);

log("An info");
//Info: An info

log("An error", "Error");
//Error: An error

Template string literals

Template strings are defined with the ` character. With template strings, the previous logging message can be written like this:

function log(message, mode= "Info"){
  console.log(`${mode}: ${message}`);

Template strings can be defined on multiple lines. However, a better option is to keep the long text messages as resources, in a database for example.

See below a function that generates an HTML that spans multiple lines:

function createTodoItemHtml(todo){
  return `<li>

Proper tail-calls

A recursive function is tail recursive when the recursive call is the last thing the function does.

The tail recursive functions perform better than non tail recursive functions. The optimized tail recursive call does not create a new stack frame for each function call, but rather uses a single stack frame.

ES6 brings the tail-call optimization in strict mode.

The following function should benefit from the tail-call optimization.

function print(from, to) 
  const n = from;
  if (n > to)  return;
  //the last statement is the recursive call 
  print(n + 1, to); 

print(1, 10);

Note: the tail-call optimization is not yet supported by major browsers.


A promise is a reference to an asynchronous call. It may resolve or fail somewhere in the future.

Promises are easier to combine. As you see in the next example, it is easy to call a function when all promises are resolved, or when the first promise is resolved.

function getTodos() { return fetch("/todos"); }
function getUsers() { return fetch("/users"); }
function getAlbums(){ return fetch("/albums"); }

const getPromises = [


function doSomethingWhenAll(){}
function doSomethingWhenOne(){}

The fetch() function, part of the Fetch API, returns a promise.

Promise.all() returns a promise that resolves when all input promises have resolved. Promise.race() returns a promise that resolves or rejects when one of the input promises resolves or rejects.

A promise can be in one of the three states: pending, resolved or rejected. The promise will in pending until is either resolved or rejected.

Promises support a chaining system that allows you to pass data through a set of functions. In the next example, the result of getTodos() is passed as input to toJson(), then its result is passed as input to getTopPriority(), and then its result is passed as input to renderTodos() function. When an error is thrown or a promise is rejected the handleError is called.


function toJson(response){}
function getTopPriority(todos){}
function renderTodos(todos){}
function handleError(error){}

In the previous example, .then() handles the success scenario and .catch() handles the error scenario. If there is an error at any step, the chain control jumps to the closest rejection handler down the chain.

Promise.resolve() returns a resolved promise. Promise.reject() returns a rejected promise.


Class is sugar syntax for creating objects with a custom prototype. It has a better syntax than the previous one, the function constructor. Check out the next exemple:

class Service {
  doSomething(){ console.log("doSomething"); }

let service = new Service();
console.log(service.__proto__ === Service.prototype);

All methods defined in the Service class will be added to theService.prototype object. Instances of the Service class will have the same prototype (Service.prototype) object. All instances will delegate method calls to the Service.prototype object. Methods are defined once onService.prototype and then inherited by all instances.


“Classes can inherit from other classes”. Below is an example of inheritancewhere the SpecialService class “inherits” from the Service class:

class Service {
  doSomething(){ console.log("doSomething"); }

class SpecialService extends Service {
  doSomethingElse(){ console.log("doSomethingElse"); }  

let specialService = new SpecialService();

All methods defined in the SpecialService class will be added to the SpecialService.prototype object. All instances will delegate method calls to the SpecialService.prototype object. If the method is not found in SpecialService.prototype, it will be searched in the Service.prototypeobject. If it is still not found, it will be searched in Object.prototype.

Class can become a bad feature

Even if they seem encapsulated, all members of a class are public. You still need to manage problems with this losing context. The public API is mutable.

class can become a bad feature if you neglect the functional side of JavaScript. class may give the impression of a class-based language when JavaScript is both a functional programming language and a prototype-based language.

Encapsulated objects can be created with factory functions. Consider the next example:

function Service() {
  function doSomething(){ console.log("doSomething"); }
  return Object.freeze({

This time all members are private by default. The public API is immutable. There is no need to manage issues with this losing context.

class may be used as an exception if required by the components framework. This was the case with React, but is not the case anymore with React Hooks.

For more on why to favor factory functions, take a look at Class vs Factory function: exploring the way forward.

Arrow functions

Arrow functions can create anonymous functions on the fly. They can be used to create small callbacks, with a shorter syntax.

Let’s take a collection of to-dos. A to-do has an id , a title , and a completed boolean property. Now, consider the next code that selects only the title from the collection:

const titles = => todo.title);

or the next example selecting only the todos that are not completed:

const filteredTodos = todos.filter(todo => !todo.completed);


Arrow functions don’t have their own this and arguments. As a result, you may see the arrow function used to fix problems with this losing context. I think that the best way to avoid this problem is to not use this at all.

Arrow functions can become a bad feature

Arrow functions can become a bad feature when used to the detriment of named functions. This will create readability and maintainability problems. Look at the next code written only with anonymous arrow functions:

const newTodos = todos.filter(todo => 
       !todo.completed && todo.type === "RE")
    .map(todo => ({
       title : todo.title,
       userName : users[todo.userId].name
    .sort((todo1, todo2) =>  

Now, check out the same logic refactored to pure functions with intention revealing names and decide which of them is easier to understand:

const newTodos = todos.filter(isTopPriority)
  .map(partial(toTodoView, users))

function isTopPriority(todo){
  return !todo.completed && todo.type === "RE";
function toTodoView(users, todo){
  return {
    title : todo.title,
    userName : users[todo.userId].name

function ascByUserName(todo1, todo2){
  return todo1.userName.localeCompare(todo2.userName);

Even more, anonymous arrow functions will appear as (anonymous) in the Call Stack.

For more on why to favor named functions, take a look at How to make your code better with intention-revealing function names.

Less code doesn’t necessary mean more readable. Look at the next exampleand see which version is easier for you to understand:

//with arrow function
const prop = key => obj => obj[key];

//with function keyword
function prop(key){
   return function(obj){
      return obj[key];

Pay attention when returning an object. In the next example, the getSampleTodo() returns undefined.

const getSampleTodo = () => { title : "A sample todo" };



I think the ES6 generator is an unnecessary feature that makes code more complicated.

The ES6 generator creates an object that has the next() method. The next() method creates an object that has the value property. ES6 generators promote the use of loops. Take a look at code below:

function* sequence(){
  let count = 0;
  while(true) {
    count += 1;
    yield count;

const generator = sequence();;//1;//2;//3

The same generator can be simply implemented with a closure.

function sequence(){
  let count = 0;
  return function(){
    count += 1;
    return count;

const generator = sequence();

For more examples with functional generators take a look at Let’s experiment with functional generators and the pipeline operator in JavaScript.


let and const declare and initialize variables.

Modules encapsulate functionality and expose only a small part.

The spread operator, rest parameter, and property shorthand make things easier to express.

Promises and tail recursion complete the functional programming toolbox.

Read Functional Architecture with React and Redux and learn how to build apps in function style.

Discover Functional JavaScript was named one of the best new Functional Programming books by BookAuthority!

For more on applying functional programming techniques in React take a look at Functional React.

You can find me on Medium and Twitter.