ES6 module syntax

Dive deep into the world of ES6 module syntax with this comprehensive article. Learn the significance of ES6 in modern web development…

Anton Ioffe · 15 minute read

As technology continues to evolve, so does the toolset used by developers, strengthening workflows, and streamlining processes. The dawn of ES6 and ECMAScript Modules has brought about a revolution in the realm of Javascript, opening up avenues of capabilities that were previously unexplored. This article aims to shed light on the intrinsic aspects of ES6 and ECMAScript Modules and their essential contribution in the modern web development arena.

The content is organized to guide you through the understanding of ES6 and ES Modules, right from its basics to creating your personalized modules. You'll then learn how to import and export these in ES6, moving on to their integration into HTML. Lastly, we'll decipher the nuances between ES6 Modules and CommonJS, helping you decide on the best fit for your development needs.

Written in an easy-to-understand style with a wealth of examples and provocative comparisons, this article is intended to infuse you with insights at every step. Each section is packed with illuminating content that promises to enhance your technical know-how, address common developer mistakes, and, ultimately, bolster your current technical repertoire to make an impactful difference in modern web development. So, get ready to dive in and immerse yourself in the nuances of ES6 modules.

Understanding ES6 and ECMAScript Modules

The Importance of ES6 in Modern Web Development

With ECMAScript 6 (often referred to as ES6 or ECMAScript 2015), the world of JavaScript programming took a huge leap forward. ES6, as the sixth edition of the ECMAScript standard, brought about a slew of updates that helped bring JavaScript into the modern era. This facelift incorporated features that ramped up the efficiency, elegance, and versatility of coding.

'Let' and 'const' enrich ES6’s variable declarations. Alongside these additions, arrow functions, promises, classes, enhanced object literals, template strings, and destructuring, have greatly augmented JS by boosting its expressiveness, thereby streamlining the coding of complicated applications.

Among ES6's crowning feature is the revelation of a new syntax for modules. This is a critical aspect we developers ought to embrace, given the vital role module architecture plays in modern web development.

// ES6 introduces 'let' and 'const'
let name = 'ES6';
const PI = 3.1415926;

// Arrow function syntax
const sum = (a, b) => {
    return a + b;
// The one-liner version underscores arrow function's implicit return for a single expression
const sum = (a, b) => a + b;

// Using Promises to handle async operations
let promise = new Promise((resolve, reject) => {
   // include further code here

// Introducing classes in JavaScript
class MyClass {
    constructor(param) {
      this.prop = param;

Introduction to ECMAScript Modules

The word "modules," in the ES6 context, speaks of the birth of a module system in JavaScript. Prior to this epoch, JS lacked native modules, compelling developers to craft them using libraries or design patterns.

At the heart of ECMAScript modules lies the principle of code encapsulation and organization. Such modules enable us to develop standalone chunks of code that can be shared and used in other modules. This invariably steers us towards creating reusable, scalable and uncoupled code snippets, proving invaluable when developing large applications.

Judging it from a broader perspective, modules serve as a mechanism to fragment our scripts into smaller, reusable chunks with a clear interface—allowing us to package and share functionality between projects efficiently. Well, isn't it impressive how a single line of code can pack a powerful punch!

In ES6, each module is a separate piece of code, destined to execute the moment it is loaded. Data encapsulated in a module's top-level scope (basically, the key elements, variables, functions, classes, etc., created in a module) are invisible to the outside world unless they have been explicitly exported. As such, this innate top-level scope aids in averting name clashes and maintains high code organization. It’s like having an extra room in your home where you can stash away all those secret ingredients that make your cooking utterly unique!

Remember: In the universe of modules, you can avoid conflicts and confusion only by explicitly importing the functionality. Unless given access, no parts of a module can be traced.

Common Coding Mistakes and Best Practices

Despite the power and flexibility that ES6 and its module syntax have brought along, a few pitfalls await the uninitiated JavaScript developer. Here are some of the most frequently encountered:

1. Unwarranted use of var instead of let and const

A cardinal mistake of ES6 newbies is the persistent use of var to declare variables, in place of let and const. Often, developers fall prey to the function scope of var, which creates a different set of rules to follow.

// Incorrect usage of 'var'
var x = 10;
if (true){
    var x = 20; // x is replaced
console.log(x); // Outputs 20

// Contrasting 'let' with 'var'
let y = 10;
if (true) {
    let y = 20;  // A new variable y is declared within the block scope.
console.log(y);  // Outputs 10

2. Ignoring the Details of Arrow Functions

Overlooking the nitty-gritty of arrow functions is another common mistake. Remember that arrow functions don't possess their own this; they inherit it from the parent scope.

function Timer() {
  this.seconds = 0;
  setInterval(function() { // 'this' refers to Global ('Window' in a browser), NOT Timer
  }, 1000);
let timer = new Timer();
setTimeout(() => {
  console.log(timer.seconds); // Outputs 0, not the expected result.
}, 1100);

// Correct usage
function Timer() {
  this.seconds = 0;
  setInterval(() => { // 'this' is a Timer instance
  }, 1000);
timer = new Timer();
setTimeout(() => {
  console.log(timer.seconds); // Outputs expected result after a second.
}, 1100);

3. Understanding the Intricacies of ECMAScript Modules

While it isn't necessary to delve deep into the specifics due to the beyond-the-scope nature of this article, there certain points to bear in mind. Clarity and a profound grasp of the workings of modules will steer you clear from common errors that developers commonly make.

By mastering ES6 and ECMAScript modules, you'll unlock the ability to structure efficient, modular, and maintainable applications. Being able to pinpoint and avoid the common pitfalls ensures the application of best coding practices which invariably leads to top-notch application development. Therefore, remember to pause and consider: are you getting the most out of ECMAScript modules? If you find yourself puzzled, don't fret. It might be the catalyst for your next adventurous coding session!

Making your own ES6 Modules

Creating Basic ES6 Modules

ES6 modules are independently executable pieces of code where important details and dependencies are managed inside its scope. Each module can expose its functionalities as exports to be consumed by other modules.

Let's use a simple function that calculates the square of a number as an example. We will create this basic ES6 module:

// module.js
export function square(num) {
    return num * num;

As you can see, prefixes export allow other modules to import this function.

Defining ES6 Modules

Defining ES6 modules is an important step towards modular and maintainable JavaScript codebase. Let's expand the earlier module.js with a new feature - cube calculation function, and this will be default export of our module:

// module.js
function square(num) {
    return num * num;

function cube(num) {
    return num * num * num;

export default cube; 
export { square };

In this example, we prefixed the function cube with export default, which means it will be the default export whenever someone imports this module. The square function will be a named export.

Common Mistake 1: A frequent mistake is trying to use export default multiple times within the same module. Remember, a module can have only one default export. Here is the wrong approach and then its correction:


function square(num) {
    return num * num;

export default function cube(num) {
    return num * num * num;

export default square; // <-- This is not allowed


function square(num) {
    return num * num;

export default function cube(num) {
    return num * num * num;

export { square }; // Correct approach

Creating Personalized ES6 Modules

You might encounter a situation where you want to build a more personalized or complex module, perhaps a module that expose multiple functions or objects. ES6 modules allow us to create such complex yet flexible modules.

Now, suppose we want to create a module that offers several mathematical operations. Observe the following code closely:

// mathOperations.js
export function square(num) {
    return num * num;

export function cube(num) {
    return num * num * num;

export function multiply(num1, num2) {
    return num1 * num2;

As you can see, we exposed several functions from this module. Each of these functions can be imported independently from the module. This enables us to create a module with many interconnected functionalities while still maintaining the cleanliness and simplicity of our codebase.

Common Mistake 2: Another common mistake is trying to export non-declared variables. JavaScript would throw a SyntaxError. Here's the erroneous code and then its correction:


export { num }; // num is not defined


let num = 10;
export { num }; // This is valid because num is declared

This concluded our in-depth analysis of creating your own ES6 modules. Hopefully, you're now more confident and curious enough to start building your personalized modules. However, remember to avoid common pitfalls and always adhere to best practices.

Do you feel ready to create your ES6 modules that can greatly enhance your ability to write clean and maintainable JavaScript code?

Congratulations! You're good to go. Stay curious, keep learning!

Importing and Exporting Modules in ES6

Importing Local Modules

In ES6 JavaScript, you utilize the import keyword to include local modules into your code. These modules might be other JavaScript files in your project or JavaScript libraries you have installed. Here is a basic syntax:

import { functionName } from './localModuleName';

In this scenario, functionName is the name of the function you wish to use from the module. It should be enclosed in curly braces, ./localModuleName is the path to the module file you're importing.

Applied in practical coding, the statement above allows you to import a function called increment from a local module named "counter". Below, you can find an example of this:

import { increment } from './counter';


Take note that local imports necessitate the use of relative file paths, denoted by a dot (.) before the slash. Common mistakes developers make include using absolute file paths or not using file paths at all. An incorrect attempt to import the function from the same module might look like this:

Incorrect Way:

import { increment } from 'counter';

The correct import statement has been given before:

Correct Way:

import { increment } from './counter';

Importing External Modules

As for external modules, the import syntax stays assortedly similar. Here, rather than refer to a local file path, you'll be referring to the name of the installed package.

import { functionName } from 'externalModuleName';

Assume you have installed an NPM package called "lodash", and you'd like to use the "join" function from it. The correct code to import "join" function from "lodash" would be:

import { join } from 'lodash';

join(['Hello', 'world'], ' ');

Exporting Modules

To make your functions, classes, or variables usable in other modules, you have to export them first. This works with the export keyword. Below is the rudimentary exporting syntax:

export function myFunction() {
    // Code here

Note: The export keyword can be placed before variable declarations, function declarations, and class declarations.

The function exported in the code snippet above can then be imported into another module using the import syntax outlined earlier. A common mistake when exporting is using parentheses.

Incorrect Way:

export (myFunction(){});

The correct way to export is:

Correct Way:

export function myFunction() {
    // Code here

ES6 Modules as Singletons

One of the interesting aspects of ES6 modules is that they act as singletons. This means that no matter how many times you import a module in your project, you'll always get the same instance of that module. Consequently, if you mutate the module in one place, those changes will affect every instance. This reduces memory footprint since every import statement points to the same object in memory. However, care must be taken as it may impact performance if the module's size is large or if the module is changed frequently.

Here is an example:

import { myObj } from './myModule';

myObj.prop = 'changed';

In this code, changing myObj.prop in one module will affect myObj.prop in all other modules where it is imported.

In conclusion, ES6 modules, with their import and export syntax, provide a powerful way to organize and manage code in your JavaScript projects. They offer modularity, reusability, and readability while being excellent performance-wise. However, it is necessary to evade common pitfalls related to their use, as failing to do so may result in errors and inconsistencies in your code.

Integrating ES6 Modules into HTML

Integrating ES6 Modules into HTML

The advent of ES6 modules in modern web development heralds the opportunity to build systems that are efficient, compartmentalized, and easy to maintain. Increasingly, developers are shifting towards integrating ES6 modules into their HTML codebase, given the enhanced experience they offer over the conventional methods of incorporating JavaScript.

ES6 Modules in HTML

The method to include ES6 modules into HTML files is quite straightforward. The 'script' element should carry a 'type' attribute with the value 'module'. This signifies the use of ES6 module syntax. The following example provides a practical demonstration:

<script type='module' src='./main.js'></script>

In this instance, the JavaScript file 'main.js' is explicitly designated as an ES6 module and duly incorporated into the HTML document.

Browser Compatibility and Backward-Compatibility Solutions

While the amenableness of modern browsers to ES6 modules is typically a given, the same cannot be said for outdated browsers. Internet Explorer, infamously known for creating compatibility issues, is a prime example. If your audience comprises laggards in the sphere of browser updates, the compatibility of ES6 modules can pose difficulties.

Solutions such as Polyfills are an effective rescue strategy. Polyfills are scripts that pave way for the functioning of modern browser features in their older counterparts. Another viable approach involves transpiling your code to ES5 through tools like Babel, ensuring graceful degradation on outdated browsers.

Common Mistakes

A common headscratcher that developers frequently encounter concerns the appropriate usage of the necessary attributes while implementing ES6 modules into HTML.

Mistake 1: Omitting type='module'
Overlooking the addition of the type='module' attribute in the script tag can disrupt the correct interpretation of the module by the browser.

Incorrect Usage

<script src='./main.js'></script>

Correct Usage

<script type='module' src='./main.js'></script>

Modular architecture is becoming ever more prominent in JavaScript development, particularly with the growing size and complexity of projects. Due to their scalability and maintainability, ES6 modules present a compelling proposition. Nevertheless, potential obstacles such as ensuring browser compatibility and navigating around syntax errors must necessarily be considered. It's important to weigh these factors as we step progressively into the era of ES6 modules. As such, how confident are you when it comes to leveraging the potential of ES6 modules? More importantly, is your team ready for this paradigm shift to a more modular approach?

ES6 Modules vs CommonJS

ES6 modules and CommonJS are two prevalent module systems in the JavaScript world. Each brings a unique syntax, usage style and advantages to the table. A suitable module system for you largely hinges on factors such as your project's scale, complexity, performance requirements, and deployment environment.

Let's dive deeper to understand the benefits and drawbacks of ES6 Modules and CommonJS, including real-world usage and common pitfalls.

ES6 Modules

ES6 Modules, standardized in the language specification, can function directly in the browser. They pave the way for static code analysis, which is beneficial for performance optimization.

Pros of ES6 Modules

  1. Static Analysis: ES6 modules are static, enabling bundlers to execute "tree shaking", which helps in the elimination of unused exports and shrinking of application size, fostering better performance.
// Only sum and pi are exported, not multiply
export const sum = (a, b) => a + b;
export const pi = 3.14;
const multiply = (a, b) => a * b;
  1. Live Bindings: Any changes in variable values within the imported modules are reflected in all the places they are used.
// counter.js
let counter = 1;
export {counter};

// app.js
import {counter} from 'counter.js';
counter++; // Changes are reflected in the imported module's counter
  1. Better Syntax: Unlike CommonJS, ES6 Modules offer a neat, declarative syntax for importing and exporting modules.
// Exporting in ES6
export function sum(a, b) {
    return a + b;

// Importing in ES6
import { sum } from './math';

Cons of ES6 Modules

  1. Async Loading: ES6 Modules lack native support for asynchronous loading.

  2. Isolation: They do not provide the same isolation level or support the export of a customized value, in the same way CommonJS's module.exports() does.

Common Mistake in ES6 Modules: Developers often use ES6 modules natively in browsers that don't fully support ES6, leading to compatibility issues.

import {sum} from 'utils.js';  // Will cause an error in non-ES6 compatible browsers

The right approach would be to transpile your ES6 code to ES5 using bundlers/transpilers, such as Babel or Webpack. This not only broadens your code compatibility but also prepares you for legacy or lower-powered devices.


CommonJS's synchronous loading pattern makes it apt for server-side scripting. It does not natively support static imports and exports, but it does allow for module.exports() for a singular custom aspect.

Pros of CommonJS

  1. Server Support: The synchronous nature of CommonJS makes it ideal for server-side scripting.

  2. Simple Syntax: The simplicity of CommonJS is apparent in the usage of the require() function for importing and exporting modules.

// Importing in CommonJS
const math = require('math.js');
// Using the exported function 
console.log(math.sum(1, 2));  
  1. Custom Exports: module.exports() feature of CommonJS lends support to the export of a single custom value.

Cons of CommonJS

  1. No Tree Shaking: The dynamic nature of require() disallows any tree shaking, which could lead to an increased bundle size.

  2. No Live Bindings: The imported values are a copy, and changes made to the original module would not affect its local copy.

Common Mistake in CommonJS: A prevalent mistake is prematurely importing a module.

// results in an error if math.js is still being loaded
const math = require('math.js');

The correct method is to ensure the module is fully loaded before consuming it, typically done using callback functions.

Summing it up, your choice between ES6 Modules and CommonJS is contingent upon your project's needs and constraints. ES6 Modules, offering benefits such as tree shaking and live bindings, are perfect for modern browsers and large-scale client-side applications. However, if your project values simplicity, server-side support, and custom exports, CommonJS could be a better pick. Additionally, keep in mind the module system's compatibility with your tools and chosen frameworks. Make a mindful choice and let your project requirements guide your decision.


This article comprehensively covers the fundamentals of ES6 and ECMAScript Modules, emphasizing their role in modern web development. You'll delve into the inherent features of ES6, the significance of ECMAScript modules, the creation of personalized modules, and their integration into the HTML. The piece also draws at the comparison of ES6 Modules with CommonJS, highlighting their pros, cons, and prevalent compatibility issues, guiding you to decide which is more reflective of your development needs.

Each section is coupled with rich examples, valuable usage tips, and illustrates the often-overlooked mistakes committed by developers. This well-knit information is destined to enhance your understanding of ES6 modules, encouraging their effective adaption in your development environment. Your task now is to critique the effectiveness of ES6 against CommonJS from your own experience as a developer. Experiment with the creation of your own module or converting a CommonJS module into ES6. Analyze how comfortable you are with the new syntax, consider how it can improve your codebase, and, most importantly, contemplate if you're ready for this paradigm shift. This exercise is sure to test your understanding and application of concepts discussed. Are you ready for it?