Why Use ECMAScript Decorators Instead of JavaScript Decorators? A Deep Dive

ECMAScript is the standard that defines JavaScript; not every JavaScript engine implements every future feature, so a feature may be at different implementation stages across browsers. TC39 governs the ECMAScript specification and a new feature passes through four stages (0‑Showcase, 1‑Proposal, 2‑Draft, 3‑Candidate, 4‑Finished) before becoming part of the standard.

Property Descriptors and Object.defineProperty

A property descriptor describes the attributes of an object property (value, writable, enumerable, configurable, get, set). Using Object.getOwnPropertyDescriptor(obj, 'prop') you can read these attributes, and Object.defineProperty(obj, 'prop', { writable: false }) can change them, for example making a property read‑only.

Multiple descriptors can be defined at once with Object.defineProperties. Creating objects without a prototype or with custom prototypes is possible via Object.create(proto, descriptors).

Method Decorators

Decorators are functions that receive the target, property name, and descriptor, allowing you to modify the descriptor before the method is defined. A simple @readonly decorator sets descriptor.writable = false. Decorators can also wrap methods to add behavior, such as logging, by storing the original method and returning a new function that calls the original.

Class Instance Field Decorators (Stage 3)

Class fields (instance properties) are a new proposal. Babel uses an initializer function in the descriptor instead of a static value. A decorator can replace the initializer to transform the default value, e.g., an @upperCase decorator that returns initValue.toUpperCase(). Parameterized decorators like @toCase('lower') can choose the transformation based on arguments.

Class Decorators

A class decorator receives the class (constructor) and must return a new constructor. It can extend the original class, add properties, or replace methods. For example, a @withLoginStatus decorator returns a subclass that adds an isLoggedIn flag and a setLoggedIn method. Multiple decorators can be stacked on a class, executing in the order they appear.

All examples assume the use of Babel plugins (e.g., babel-plugin-transform-decorators-legacy and babel-plugin-transform-class-properties) to transpile the experimental syntax.

var myObj = { myPropOne: 1, myPropTwo: 2 };</code>
<code>Object.defineProperty(myObj, 'myPropOne', { writable: false });</code>
<code>Object.defineProperty(User.prototype, 'getFullName', { writable: false });</code>
<code>function readonly(target, property, descriptor) { descriptor.writable = false; return descriptor; }</code>
<code>@readonly
class User { getFullName() { return `${this.firstName} ${this.lastName}`; } }

These techniques enable developers to write more expressive, maintainable, and secure JavaScript code while awaiting full standardization of decorators.