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Ivan Rojas

JavaScript ES3, ES6, and ES9: Understanding Key Versions

Explore the key milestones in JavaScript's history by comparing features from ECMAScript versions ES3 (1999), the revolutionary ES6 (2015), and the refining ES9 (2018). Essential knowledge for modern developers.
Discover JS Version Features
JavaScript, the dynamic language driving much of the modern web, evolves through the ECMAScript (ES) standard. Understanding its key versions helps developers write effective code and navigate the ecosystem used widely in Canada and globally.
While new versions now release annually, certain milestones mark significant shifts. ES3 represented the long-established baseline, ES6 (ES2015) brought a massive modernization, and ES9 (ES2018) added further important capabilities.
This guide contrasts the landscape defined by ES3 with the major advancements of ES6 and the subsequent refinements introduced in ES9.
Conceptual image of older computer screen showing basic code

ES3 (1999): The Longstanding Foundation

For years, ES3 was the standard for web scripting. It provided core syntax, `var` for variables (with function scope), prototype-based objects, regular expressions, and basic error handling (`try/catch`).
Modern code editor interface displaying clean ES6+ JavaScript syntax

ES6 (ES2015): Ushering in Modern JavaScript

This pivotal update introduced features essential for contemporary development: block-scoped `let` and `const`, concise arrow functions, `class` syntax, native modules (`import`/`export`), Promises for async operations, and more readable string handling with template literals.
Code snippet illustrating ES9 features like object spread or async iteration

ES9 (ES2018): Advancing Asynchronous & Object Handling

Building on the modern foundation, ES9 delivered valuable additions including asynchronous iterators (`for-await-of`), convenient rest/spread properties for objects (`...`), `Promise.prototype.finally()`, and powerful updates to regular expressions.
Browser logos (Chrome, Firefox, Safari, Edge) alongside a Babel logo

Version Evolution & Browser Compatibility

ECMAScript now evolves annually. Modern browsers boast excellent support for ES6 and ES9. However, for compatibility with older environments, developers often use transpilers like Babel to convert newer code to older, more widely supported versions (like ES5).
Developer working intently on JavaScript code on a computer

Significance for Today's Developers

Proficiency in ES6+ is standard for modern JavaScript roles in Canada and worldwide. Understanding ES9 features provides useful tools. Knowledge of ES3's limitations clarifies the 'why' behind modern features and the need for tools like Babel.
Developer choosing between different modern JavaScript tools or features

Adopting Modern Features Wisely

Developers today generally leverage ES6 and later features. The decision to use specific newer syntax often depends on project needs, team standards, browser support targets, and the project's build/transpilation process.
Stylized graphic showing the progression and growth of the JavaScript language

JavaScript: A Continuously Evolving Language

The evolution from ES3 through ES6 and ES9 demonstrates JavaScript's journey towards becoming a more robust, developer-friendly, and powerful language capable of handling complex modern applications.
Recognizing these major shifts helps developers write more effective code and stay current with the language's ongoing development via the ECMAScript standard.
Understanding these core concepts helps navigate JavaScript versions.

ECMAScript Standard

  • Official specification for JavaScript language.
  • Managed by ECMA International (TC39).
  • Defines syntax, types, objects, features.
  • Versions: ES3, ES5, ES6(2015), ES9(2018), etc.
  • Ensures cross-browser consistency goal.

JavaScript Engines

  • Implement the ES standard in browsers/Node.js.
  • Examples: V8, SpiderMonkey, JavaScriptCore.
  • Parse and execute JS code.
  • Implement new ES features over time.
  • Engine choice impacts performance.

ES6 (ES2015) Baseline

  • Marked shift to "modern" JavaScript.
  • Introduced block scope, classes, modules.
  • Simplified async with Promises.
  • Foundation for current development practices.
  • Excellent support in modern browsers.

ES9 (ES2018) Additions

  • Followed annual release cadence.
  • Added async iteration capabilities.
  • Object rest/spread syntax introduced.
  • `Promise.prototype.finally` method.
  • Significant RegExp improvements.

Backwards Compatibility

  • New versions aim not to break existing code.
  • Features are added, not removed (mostly).
  • Helps ensure the web doesn't break.
  • Core principle of ECMAScript evolution.
  • Transpilers bridge gap for new syntax.

Transpilation (Babel)

  • Converts new JS syntax to older syntax.
  • Tool like Babel widely used.
  • Ensures code runs in older environments.
  • Allows use of modern features safely.
  • Common part of build toolchains.
Graphic showing a bridge between old and new JavaScript code
ES6 (ES2015) represents the great divide, introducing features that fundamentally improved JavaScript's structure, readability, and capabilities for building complex software.

Key Features Introduced in ES6 & ES9

ES6: let & const

Block-scoped variable declarations.

ES6: Arrow Functions

Concise syntax, lexical `this`.

ES6: Classes

Simpler object-oriented syntax.

ES6: Modules

Native `import`/`export` system.

ES6: Template Literals

Easy string interpolation (`` ` ``).

ES6: Promises

Standard for handling async code.

ES9: Async Iteration

Using `for-await-of` loops.

ES9: Object Rest/Spread

Ellipsis (`...`) for object properties.

ES9: Promise.finally()

Runs code after promise settles.

ES9: RegExp Lookbehind

Matching based on preceding text.

ES9: RegExp Named Groups

Accessing capture groups by name.

ES9: RegExp dotAll Flag

Makes `.` match newline characters.

JavaScript Version FAQs

What is ECMAScript (ES) versus JavaScript?
ECMAScript is the official specification or standard for scripting languages, maintained by ECMA International. JavaScript is the most well-known and widely used implementation of the ECMAScript standard. The terms are often used interchangeably when discussing language features (e.g., ES6 features are JavaScript features).
Why was ES6 (ES2015) such a big deal?
ES6 introduced fundamental features missing in earlier versions, such as block scope ('let'/'const'), classes, modules, Promises, and arrow functions. These additions significantly modernized the language, making it more suitable for large-scale application development and improving developer experience.
Key features of ES9 (ES2018)?
Significant ES9 features include asynchronous iteration ('for-await-of'), rest/spread properties for objects ('...'), Promise.prototype.finally(), and several useful enhancements to regular expressions (like lookbehind assertions and named capture groups).
Do I need to know ES3 now?
For new development targeting modern browsers, generally no. However, understanding ES3 basics (like 'var' scope) helps with legacy codebases or understanding why certain modern features (and tools like Babel) exist.
Is ES9 fully supported by browsers now (in 2025)?
Yes, current versions of major browsers (Chrome, Firefox, Safari, Edge) have excellent support for ES9 (ES2018) features. Compatibility issues are rare outside of very old or niche environments.
What does Babel do?
Babel is a widely used JavaScript transpiler. It compiles modern JavaScript code (ES6+) into older, more broadly compatible versions (typically ES5) so developers can use new features while ensuring the code runs in older browsers or environments.
Briefly, let vs. const vs. var?
'var' (pre-ES6) is function-scoped. 'let' and 'const' (ES6+) are block-scoped. 'let' variables can be reassigned; 'const' variables cannot be reassigned (though their internal state, if an object or array, can be modified).
Main benefit of arrow functions (=>)?
They offer a shorter syntax and, importantly, do not have their own 'this' binding—they inherit 'this' from their surrounding scope, simplifying code and avoiding common issues found with traditional functions in certain contexts.
Recall the foundation ES3 (1999): The Baseline:
• `var` keyword (function scope)
• Prototype-based inheritance
• Core syntax established (try/catch, regex)
• Foundation for early web interactivity
The long-standing standard before major modernizations.
Embrace modernization ES6/ES2015: The Revolution:
• `let`/`const` (block scope)
• Arrow functions (`=>`)
• Classes & Modules introduced
• Promises for async handling
Fundamentally changed JavaScript, enabling modern development practices.
Integrate refinements ES9/ES2018: Key Additions:
• Async iteration (`for-await-of`)
• Object rest/spread properties (`...`)
• `Promise.prototype.finally()`
• RegExp enhancements
Added powerful features for asynchronous programming and object manipulation.
Ensure browser support Understanding Compatibility:
• Modern browsers support ES6/ES9 well
• Older environments may need transpiling
• Babel commonly used tool
• Consider target audience's browsers
Consider target environments when using newer ES features or use transpilers.
Appreciate language growth The ECMAScript Evolution:
• ES standard evolves annually post-ES6
• Each version adds features/refinements
• Understanding history aids comprehension
• Keeps JS relevant and powerful
JavaScript is a living language with ongoing improvements via ECMAScript.
Apply modern practices Developer Relevance Today:
• ES6+ knowledge is standard expectation
• Understanding older ES3 helps legacy code
• Choose features appropriate for project needs
• Continuous learning of ES updates vital
Knowledge of ES versions is crucial for effective modern JavaScript development in Canada and globally.

JavaScript's Journey: Key ES Milestones

Understanding the evolution of JavaScript through its ECMAScript (ES) standard versions provides valuable context for any web developer. Key milestones like ES3, ES6, and ES9 represent significant shifts in the language's capabilities.
ES3 established the long-running foundation many developers worked with for years, setting core syntax and functionality.
ES6 (or ES2015) was a transformative update, introducing features that modernized the language and enabled more complex, maintainable application development.
ES9 (ES2018) continued this progress, adding powerful refinements, particularly for asynchronous operations and object handling, reflecting the annual enhancement cycle adopted after ES6.
Timeline or graphic showing progression of JavaScript versions

ES3 (ECMAScript 3 - 1999): The Foundation

Published in 1999, ES3 served as the dominant JavaScript standard for over a decade. It solidified many core language features still used today.
Key characteristics included variable declaration using `var`, which notably has function scope, not block scope. It relied on prototype-based inheritance for object-oriented patterns.
ES3 introduced essential functionalities like regular expressions for pattern matching and the `try...catch` statement for handling errors gracefully.
While foundational, it lacked features now considered standard, such as block-scoped variables (`let`, `const`), native classes, modules for code organization, and streamlined asynchronous programming tools like Promises or async/await.

ES6 (ECMAScript 2015): Ushering in Modern JavaScript

ES6, officially ECMAScript 2015, represented a watershed moment for JavaScript, introducing a vast array of features that fundamentally changed how developers write code.
Major Syntactic Improvements: Introduced `let` and `const` for block-scoped variable declarations, addressing issues with `var`. Arrow functions (`=>`) provided a more concise function syntax with lexical `this` binding. `class` syntax offered a clearer way to work with objects and inheritance (though still prototype-based underneath). Template literals (backticks `` ` ``) simplified string interpolation and multi-line strings.
Enhanced Functionality: Native Modules (`import`/`export`) allowed for better code organization and reuse without external libraries. Promises provided a standard way to handle asynchronous operations, mitigating "callback hell". Destructuring assignment offered easier extraction of values from arrays and objects. Default function parameters and rest/spread operators (`...`) added flexibility.
ES6 became the new baseline for modern JavaScript development, widely adopted across browsers and Node.js (often with transpilation for older targets).

ES9 (ECMAScript 2018): Advancing Modern Capabilities

Following the shift to annual releases after ES6, ES9 (ES2018) introduced several important features and refinements building upon the modern foundation.
Asynchronous Enhancements: The introduction of asynchronous iteration (`for-await-of`) provided a cleaner syntax for looping over asynchronous data sources (like streams or APIs that return data piece by piece). `Promise.prototype.finally()` added a way to execute code after a Promise settles, regardless of whether it resolved or rejected.
Object Manipulation: Rest and Spread properties (`...`) were extended to work with objects, simplifying tasks like copying object properties or collecting remaining properties during destructuring.
Regular Expression Improvements: ES9 brought significant upgrades to RegExp, including lookbehind assertions (matching based on preceding text), named capture groups (accessing matched groups by name), the `s` (dotAll) flag (allowing `.` to match newline characters), and Unicode property escapes.
These ES9 features further enhanced JavaScript's capabilities for handling complex asynchronous flows, data manipulation, and text processing.

Why Understanding ES Versions Matters in 2025

In today's development landscape (including Canada's vibrant tech sector), a solid grasp of ES6+ features is considered standard for JavaScript developers. Most new projects utilize these modern capabilities extensively.
Knowledge of ES9 features, particularly async iteration and object spread/rest, provides powerful tools for common development tasks. Since ECMAScript now evolves annually, staying aware of newer features beyond ES9 is also part of continuous learning.
Understanding the history, including the limitations of ES3 and the revolutionary impact of ES6, helps developers appreciate *why* modern features exist, how to work with legacy code effectively, and when tools like Babel (transpilers) are necessary to ensure compatibility with older browsers or environments.
A well-rounded JavaScript developer understands not just the latest syntax, but the evolution that led to it, enabling more informed and effective coding practices.

What is the official standard that JavaScript implements?

ECMAScript (ES).

Which ES version revolutionized JavaScript in 2015?

ES6 (also known as ES2015).

Which ES6 keyword declares a block-scoped variable that can be reassigned?

`let`. (`const` is block-scoped but cannot be reassigned).

What ES6 feature improved asynchronous programming?

Promises (later enhanced by async/await in ES7/ES2017).

What tool converts modern JavaScript for older browsers?

A transpiler, like Babel.

Which ES version introduced object rest/spread properties (`...`)?

ES9 (ES2018).