# The Difference Between JavaScript, ECMAScript, and ES6/ES2015: A Complete Guide

> Understand the difference between JavaScript, ECMAScript, and ES6. Learn how this specification drives modern web development and explore key features of ES2015.

- Repository: [Kyle Simpson/You-Dont-Know-JS](https://github.com/getify/You-Dont-Know-JS)
- Tags: deep-dive
- Published: 2026-02-24

---

**JavaScript is the runtime implementation of the ECMAScript specification, while ES6 (also called ES2015) is the sixth edition of that specification published in 2015.**

The terminology surrounding JavaScript confuses even experienced developers. According to the *You Don't Know JS* repository by getify, understanding the difference between JavaScript, ECMAScript, and ES6/ES2015 is fundamental to mastering the language ecosystem. This article breaks down exactly how these terms relate based on the authoritative source code analysis of the popular open-source book series.

## What Is ECMAScript?

ECMAScript is the **formal specification** that defines the JavaScript language core. Maintained by the TC39 committee and published by ECMA International, the standard is versioned by year (e.g., ECMAScript 2015, ECMAScript 2022) and describes the exact syntax, semantics, and built-in objects that any conforming engine must implement.

As noted in [`get-started/ch1.md`](https://github.com/getify/You-Dont-Know-JS/blob/main/get-started/ch1.md), "the official name of the language specified by TC39 … is **ECMAScript**" [L42]. The specification exists independently of any runtime environment, serving as the technical blueprint that browser vendors and JavaScript engine developers follow to ensure language consistency across platforms.

## What Is JavaScript?

JavaScript represents both the **brand name** and the **runtime implementations** that execute ECMAScript code. While browsers, Node.js, Deno, and other environments expose the same language surface defined by the ECMAScript spec, they also provide host-specific APIs (such as the DOM in browsers or `process` in Node.js) that fall **outside** the ECMAScript specification.

In [`get-started/ch1.md`](https://github.com/getify/You-Dont-Know-JS/blob/main/get-started/ch1.md) [L42], the author clarifies that JavaScript is essentially the implementation that runs in browsers and other engines. When developers refer to "JavaScript," they typically mean the runtime environment including these host-specific extensions, not just the pure language syntax defined by ECMAScript.

## Understanding ES6 and ES2015

ES6 and ES2015 refer to the **same specification edition**—the sixth version of the ECMAScript standard, published in 2015. This edition introduced transformational language features that modern JavaScript development relies upon today.

According to [`objects-classes/ch1.md`](https://github.com/getify/You-Dont-Know-JS/blob/main/objects-classes/ch1.md), ES6 is "the 2015 edition of the spec that added dedicated syntax like `class` and `extends`" [L12]. The dual naming convention (ES6 versus ES2015) exists because TC39 shifted from edition numbers to year-based versioning after this release, though "ES6" remains common in developer vocabulary.

### Key Features Introduced in ES6

The ES6/ES2015 specification added substantial capabilities to the language core:

- **`let` and `const`** for block-scoped variable declarations
- **Arrow functions** (`=>`) providing concise syntax and lexical `this` binding
- **Class syntax** with `class`, `extends`, and `constructor` keywords
- **Modules** (`import`/`export`) for standardized code organization
- **New built-in objects** including `Map`, `Set`, `Promise`, and `Symbol`

The arrow function implementation is specifically discussed in [`scope-closures/ch3.md`](https://github.com/getify/You-Dont-Know-JS/blob/main/scope-closures/ch3.md) [L343], while the module system is covered in [`get-started/ch2.md`](https://github.com/getify/You-Dont-Know-JS/blob/main/get-started/ch2.md) [L34].

## Practical Code Examples

These features demonstrate the relationship between the specification and implementation:

```js
// ES6 syntax defined by ECMAScript 2015 spec
let count = 0;                     // Block-scoped variable
const PI = 3.14159;                // Immutable constant declaration
const add = (a, b) => a + b;       // Arrow function (scope-closures/ch3.md L343)

// Class syntax introduced in ES6 (objects-classes/ch1.md L12)
class Person {
  constructor(name) {
    this.name = name;
  }
  greet() {
    console.log(`Hello, ${this.name}!`);
  }
}

// ES6 modules (get-started/ch2.md L34)
export const utils = { add };

```

Every engine executing this code implements the **ECMAScript 2015 specification**, while the specific runtime (browser console, Node.js process, etc.) constitutes the **JavaScript** environment.

## Summary

- **ECMAScript** is the formal language specification maintained by TC39 and ECMA International
- **JavaScript** is the runtime implementation (browsers, Node.js) that executes ECMAScript code plus host-specific APIs
- **ES6/ES2015** is the sixth edition of the ECMAScript specification, published in 2015, introducing `let`/`const`, arrow functions, classes, and modules
- Modern development typically targets "ES6+" or "ECMAScript 2015+" to indicate compatibility with these and subsequent features

## Frequently Asked Questions

### Is JavaScript the same as ECMAScript?

No. JavaScript is an implementation of the ECMAScript specification. While JavaScript engines strive for ECMAScript compliance, the term "JavaScript" generally includes host environment APIs (like the DOM or Node.js `process` object) that exist outside the ECMAScript standard. The specification defines the language syntax and core objects, while JavaScript refers to the actual runtime executing that code.

### What does ES6 stand for?

ES6 stands for **ECMAScript 6**, indicating the sixth edition of the specification. After ES6, TC39 switched to year-based naming, making ES6 and ES2015 synonymous references to the same 2015 specification release that introduced classes, arrow functions, and block-scoped declarations.

### Should I use ES6 or ES2015 when describing modern JavaScript?

Both terms are technically correct, but **ES2015** aligns with the official year-based naming convention adopted by TC39 after the sixth edition. However, "ES6" remains widely recognized in tooling configurations (like `.eslintrc` or [`tsconfig.json`](https://github.com/getify/You-Dont-Know-JS/blob/main/tsconfig.json) target settings) and developer discussions. For editions after 2015, always use the year format (ES2016, ES2017, etc.).

### Are all JavaScript engines ECMAScript compliant?

Modern JavaScript engines (V8, SpiderMonkey, JavaScriptCore) aim for strict ECMAScript compliance, but implementation timing varies. Engines may support specific features from newer specifications before the standard is finalized, or lag behind in implementing certain edge cases. The *You Don't Know JS* series emphasizes that understanding the specification helps developers predict behavior across different JavaScript runtimes regardless of compliance timing.