Understanding JavaScript decorators

Editor’s note: This post was reviewed 11 February 2022 to update code and include information about the latest breaking changes to Babel.

Introduction

According to the Cambridge dictionary, to decorate something means “to add something to an object or place, especially in order to make it more attractive.”

Decorating in programming is simply wrapping one piece of code with another, thereby decorating it. A decorator (also known as a decorator function) can additionally refer to the design pattern that wraps a function with another function to extend its functionality.

This concept is possible in JavaScript because of first-class functions — JavaScript functions that are treated as first-class citizens.

The concept of decorators is not new in JavaScript because higher-order functions are a form of function decorators.

Let’s elaborate on this in the next section, or jump to your section of interest below:

• Function decorators
  • Higher-order functions
• Class decorators
  • The JavaScript decorators API
  • Class member decorators
  • Class decorators
• Why decorators?

Function decorators

Function decorators are functions. They take a function as an argument and return a new function that enhances the function argument without modifying it.

Higher-order functions

In JavaScript, higher-order functions take a first-class function as an argument and/or return other functions.

Consider the code below:

const logger = (message) => console.log(message)

function loggerDecorator (logger) {
    return function (message) {
        logger.call(this, message)
        console.log("message logged at:", new Date().toLocaleString())
    }
}

const decoratedLogger = loggerDecorator(logger);

We have decorated the logger function by using the loggerDecorator function. The returned function — now stored in the decoratedLogger variable —  does not modify the logger function. Instead, the returned function decorates it with the ability to print the time a message is logged.

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Consider the code below:

logger("Lawrence logged in: logger") // returns Lawrence logged in: logger

decoratedLogger("Lawrence logged in: decoratedLogger") 
// returns:
// Lawrence logged in: decoratedLogger
// message logged at: 6/20/2021, 9:18:39 PM

We see that when the logger function is called, it logs the message to the console. But when the decoratedLogger function is called, it logs both the message and current time to the console.

Below is another sensible example of a function decorator:

//ordinary multiply function
let Multiply = (...args) => {
    return args.reduce((a, b) => a * b)
}

// validated integers
const Validator = (fn) => {
  return function(...args) {
    const validArgs = args.every(arg => Number.isInteger(arg));
    if (!validArgs) {
      throw new TypeError('Argument cannot be a non-integer');
    }
    return fn(...args);
  }
}

//decorated multiply function that only multiplies integers
MultiplyValidArgs = Validator(Multiply);
MultiplyValidArgs(6, 8, 2, 10);

In our code above, we have an ordinary Multiply function that gives us the product of all its arguments. However, with our Validator function — which is a decorator — we extend the functionality of our Multiply function to validate its input and multiply only integers.

Class decorators

In JavaScript, function decorators exist since the language supports higher-order functions. The pattern used in function decorators cannot easily be used on JavaScript classes. Hence, the TC39 class decorator proposal. You can learn more about the TC39 process here.

The TC39 class decorator proposal aims to solve this problem:

function log(fn) {
  return function() {
    console.log("Logged at: " + new Date().toLocaleString());
    return fn();
  }
}
class Person {
  constructor(name, age, job) {
    this.name = name;
    this.age = age;
    this.job = job;
  }
  getBio() {
    return `${this.name} is a ${this.age} years old ${this.job}`;
  }
}

// creates a new person
let man = new Person("Lawrence", 20, "developer");

// decorates the getBio method
let decoratedGetBio = log(man.getBio); 
decoratedGetBio(); // TypeError: Cannot read property 'name' of undefined at getBio

We tried to decorate the getBio method using the function decorator technique, but it does not work. We get a TypeError because when the getBio method is called inside the log function, the this variable refers the inner function to the global object.

We can work around this by binding the this variable to the man instance of the Person class as seen below:

// decorates the getBio method
let decoratedGetBio = log(man.getBio.bind(man));

decoratedGetBio(); // returns
// Logged at: 6/22/2021, 11:56:57 AM
// Lawrence is a 20 years old developer

Although this works, it requires a bit of a hack and a good understanding of the JavaScript this variable. So there is a need for a cleaner and easier-to-understand method of using decorators with classes.

Class decorators — or strictly decorators — are a proposal for extending JavaScript classes. TC39 is currently a stage 2 proposal, meaning they are expected to be developed and eventually included in the language.

However, with the introduction of ES2015+, and as transpilation has become commonplace, we can use this feature with the help of tools such as Babel by using the @babel/plugin-proposal-decorators.

Also, it is important to note that the stage 2 decorator proposal has a new and stable implementation to handle multiple constraints and desires. And the new semantics are different from the old proposal — decorators already written for Babel legacy or TypeScript will not work with the new proposal.

According to the Babel-rewrite documentation, to support this new implementation, we need to pass the "version": "2021-12" option to the @babel/plugin-proposal-decorators as seen below:

{
  "plugins": [
    ["@babel/plugin-proposal-decorators", {
        "version": "2021-12"
    }]
  ]
}

Note: for development purposes you can use the new decorators playground.

The JavaScript decorators API

Decorators use a special syntax whereby they are prefixed with an @ symbol and placed immediately above the code being decorated, as seen below:

@log
class ExampleClass {
  doSomething() {
    //
  }
}

Also, class decorators come after export and default, and it is possible to decorate both a class declaration and class expression.

When a decorator is called, it receives two arguments: value and context. The value argument refers to the value being decorated but it is undefined if it is a class field and the context refers to an object that contains metadata about the value being decorated.

In the new implementation, decorators support classes and public, private, and static class members such as methods, accessors, and class fields.

A simple implementation is shown in the code below:

function decorator (value, context) {
  console.log("decorated value is:", value);
  console.log("context is: ", context);
}

@decorator
class C {
  @decorator // decorates a class field
  p = 5;

  @decorator // decorates a method
  m() {}

  @decorator // decorates a getter
  get x() {}

  @decorator // decorates a setter
  set x(v) {}
}

Let’s learn more about the different types of decorators with the detailed examples given below:

Class member decorators

A class member decorator is a binary function applied to members of a class.

The first argument, value, refers to the member property of the class we are decorating. This makes possible a pattern where we can optionally return a new method or replace the decorated function.

If we return a new method, it will replace the original on the prototype, but if it is a static method, it will replace it on the class itself. However, if we return any other type of value, an error will be thrown.

Let’s learn about this by rewriting our log decorator:

function log(value, {kind, name}) {
  if (kind === "method") {
    return function(...args) {
      console.log("Logged at: " + new Date().toLocaleString());
      try {
        const result = value.apply(this, args);
        return result;
      } catch (e) {
        console.log(`Error: ${e}`);
        throw e;
      } 
    } 
  }  
}

class Person {
  constructor(name, age, job) {
    this.name = name;
    this.age = age;
    this.job = job;
  }
  
  @log
  getBio() {
    return `${this.name} is a ${this.age} years old ${this.job}`;
  }
}

// creates a new person
let man = new Person("Lawrence", 20, "developer");

man.getBio()

In the code above, we have successfully refactored our log decorator — from function decorator pattern to member class decorator.

We simply accessed the member class property — in this case, the getBio method — with the descriptor value, and replaced it with a new function.

This is cleaner and can be more easily reused than plain higher-order functions.

Class decorators

These decorators are applied to the whole class, enabling us to decorate the class.

The class decorator receives a class as the first argument. And it can optionally return a new class or replace the decorated class, but it throws an error if a nonconstructable value is returned.

Consider the code below:

function log(value, { kind, name }) {
  if (kind === "class") {
    const newClass = class extends value {
      constructor(...args) {
        super(...args);
        console.log(`constructing a class with arguments: ${args.join(", ")}`);
      }
    }
      console.log(`An instance of the ${name} ${kind} has been created`)
      return newClass;
  }
}

@log
class Person {
  constructor(name, profession) {
  }
}
const lawrence = new Person('Lawrence Eagles', "Developer");

In our small, contrived example, the log decorator receives the class as the first argument and logs information of an instance whenever a new class is created.

Why decorators?

Decorators enable us to write cleaner code by providing an efficient and understandable way of wrapping one piece of code with another. It also provides a clean syntax for applying this wrapper.

This syntax makes our code less distracting because it separates the feature-enhancing code away from the core function. And it enables us to add new features without increasing our code complexity.

Additionally, decorators help us extend the same functionality to several functions and classes, thereby enabling us to write code that is easier to debug and maintain.

While decorators already exist in JavaScript as higher-order functions, it is difficult or even impossible to implement this technique in classes. Hence, the special syntax TC39 offers is for easy usage with classes.

Conclusion

Although decorators are a stage 2 proposal, they are already popular in the JavaScript world — thanks to Angular and TypeScript.

From this article, we can see that they foster code reusability, thereby keeping our code DRY.

The new implementation enables us to decorate a class and everything within the class including the new class auto-accessors.

As we wait for decorators to be officially available in JavaScript, you can start using them by using Babel. And I believe you have learned enough in this article to give decorators a try in your next project.

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