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Decorator Example

Decorators are a convenient way to wrap properties and behaviour with extra functionality.

Decorators as a design pattern have been around since the original GoF design patterns

They have more in common with Java annotations than with the decorator pattern

Java annotations: my BLOG:

They are more similar to decorators in Python (and annotations in Java), which are syntactic sugar for simple wrappers

In Java, annotations can be used at compile time and at runtime to apply extra behaviours to a class or property. Python has a similar metaprogramming interface which has inspired the javascript implementation proposed by wycats

Decorators are useful for adding extra functionality to behaviours and properties that would otherwise look like boilerplate -- such as cacheing, access control, logging, instrumentation.

Getting started

Since decorators are currently in the proposal stage (, getting started requires a little tweaking of your standard babel/webpack/linter configs.


> npm install -g webpack
> npm install --save-dev babel-core babel-loader babel-preset-es2015 babel-plugin-transform-decorators-legacy

in your .babelrc

  "presets": [
  "plugins": [


here is a barebones webpack.config.js

var path = require('path')

module.exports = {
  entry: './index.js',
  output: {
    path: path.resolve(__dirname, 'build'),
    filename: 'main.js'
  module: {
      loaders: [
      test: /\.js$/,
      exclude: /node_modules/,
      loader: 'babel-loader',
      query: {
        cacheDirectory: true,
        plugins: [
        presets: ['es2015'],
  stats: {
    colors: true

the important bit is to add 'transform-decorators-legacy' to the plugins array.

Linting error -- or is it? 🤔

If you use VSCode, you will probably run across a linter error that says

[js] Experimental support for decorators is a feature that is subject to change in a future release. Set the 'experimentalDecorators' option to remove this warning.

This is an error in the VSCode JS support, rather than a linter error.

Add a jsconfig.json file to your project root with the following contents:

  "compilerOptions": {
    "experimentalDecorators": true

Be sure to restart VSCode, and the problem should go away. If it doesn't, follow this thread for further information.

How does it work?

A decorator function runs before the object it decorates is installed on the prototype. When you define an undecorated object like this:

class ExampleWithoutDecoration {
  doWork() {
    console.log('can\'t you see I\'m working here?')

The Javascript engine creates an object (ExampleWithoutDecoration) and installs the doWork method on its prototype:

Object.defineProperty(ExampleWithoutDecoration.prototype, 'doWork', {
  value: specifiedFunction,
  enumerable: false,
  configurable: true,
  writable: true

When you define a DECORATED method like this:

class ExampleWithDecoration {

  doWork() {
    console.log('can\'t you see I\'m working here?')

The Javascript engine saves some temporary state, runs the decorator function, and then installs the doWork method on the object's prototype.

let methodDescription = {
  type: 'method',
  initializer: () => specifiedFunction,
  enumerable: false,
  configurable: true,
  writable: true

methodDescription = DecoratingIsFun(ExampleWithDecoration.prototype, 'doWork', methodDescription) || methodDescription

defineDecoratedProperty(ExampleWithDecoration.prototype, 'doWork', methodDescription);

function defineDecoratedProperty(target, { initializer, enumerable, configurable, writable }) {
  Object.defineProperty(target, { value: initializer(), enumerable, configurable, writable })

In this case, the DecoratingIsFun method is run with this set to the object prototype, and it has the opportunity to modify/return a methodDescription, or use the previously specified methodDescription.

Consider DecoratingMakesSense, which makes the doWork method non-writeable:

const DecoratingMakesSense = (object, methodName, description) => {
  console.log('Decorating makes sense')

  description.writable = false
  return description

class ExampleWithDetailedDecoration {
  doWork() {
    console.log('can\'t you see I\'m working here?')

const makesSense = new ExampleWithDetailedDecoration()
makesSense.doWork = () => console.log('some other function')
> node build/main.js
Decorating makes sense
can't you see I'm working here?

makesSense.doWork = function () {

TypeError: Cannot assign to read only property 'doWork' of object '#<ExampleWithDetailedDecoration>'
    at Object.defineProperty.value (/Users/kleonard/newrelic/decorator-example/build/main.js:164:19)
    at __webpack_require__ (/Users/kleonard/newrelic/decorator-example/build/main.js:20:30)
    at i (/Users/kleonard/newrelic/decorator-example/build/main.js:63:18)
    at Object.<anonymous> (/Users/kleonard/newrelic/decorator-example/build/main.js:66:10)
    at Module._compile (module.js:570:32)
    at Object.Module._extensions..js (module.js:579:10)
    at Module.load (module.js:487:32)
    at tryModuleLoad (module.js:446:12)
    at Function.Module._load (module.js:438:3)
    at Module.runMain (module.js:604:10)

Writing your decorator

Since a decorator is a function that runs before the object, property, or method is installed

Build and run

> webpack
Hash: 63cf378bd6d165758ed8
Version: webpack 3.8.1
Time: 457ms
  Asset     Size  Chunks             Chunk Names
main.js  4.47 kB       0  [emitted]  main
   [0] ./index.js 1.7 kB {0} [built]
   [1] ./decorator.js 195 bytes {0} [built]

> node build/main.js
Decorating is fun
can't you see I'm working here?