Apa itu Metaprogramming di JavaScript? Tolong gunakan bahasa inggris.

JavaScript memiliki banyak fitur berguna yang diketahui oleh sebagian besar pengembang. Pada saat yang sama, ada beberapa permata tersembunyi yang dapat menyelesaikan masalah yang sangat menantang jika Anda menyadarinya.

Metaprogramming dalam JavaScript adalah salah satu konsep yang mungkin belum banyak kita kenal. Pada artikel ini, kita akan belajar tentang Metaprogramming dan bagaimana kegunaannya bagi kita.

Dengan ES6 (ECMAScript 2015), kami memiliki dukungan untuk objek Reflectdan Proxyyang memungkinkan kami melakukan Metaprogramming dengan mudah. Di artikel ini, kita akan mempelajari cara menggunakannya dengan contoh.

Apa itu Metaprogramming?

Metaprogrammingtidak kurang dari keajaiban dalam pemrograman ! Bagaimana dengan menulis program yang membaca, memodifikasi, menganalisis, dan bahkan menghasilkan program? Bukankah itu terdengar ajaib dan kuat?

Inilah cara saya menggambarkan Metaprogramming sebagai pengembang yang menggunakannya sepanjang waktu:

Metaprogrammingadalah teknik pemrograman di mana program komputer memiliki kemampuan untuk memperlakukan program lain sebagai datanya. Ini berarti bahwa program dapat dirancang untuk membaca, menghasilkan, menganalisis, atau mengubah program lain, dan bahkan memodifikasi dirinya sendiri saat berjalan.

Sederhananya, Metaprogramming melibatkan penulisan kode yang bisa

  • Hasilkan kode
  • Memanipulasi konstruksi bahasa pada waktu proses. Fenomena ini dikenal sebagai Reflective Metaprogrammingatau Reflection.

Apa itu Refleksi dalam Metaprogramming?

Reflectionadalah cabang dari Metaprogramming. Refleksi memiliki tiga sub-cabang:

  1. Introspeksi : Kode mampu memeriksa dirinya sendiri. Ini digunakan untuk menemukan informasi tingkat rendah tentang kode.
  2. Modifikasi Sendiri : Seperti namanya, kode dapat memodifikasi dirinya sendiri.
  3. Syafaat : Bertindak atas nama orang lain. Ini dapat dicapai dengan membungkus, menjebak, mencegat.

ES6 memberi kita Reflectobjek (alias, Reflect API) untuk dicapai Introspection. The Proxyobyek ES6 membantu kita dengan Intercession. Kami tidak akan berbicara terlalu banyak   Self-Modificationkarena kami ingin menjauh darinya sebanyak mungkin.

Tunggu sebentar! Untuk memperjelas, Metaprogramming tidak diperkenalkan di ES6. Sebaliknya, itu telah tersedia dalam bahasa sejak awal. ES6 membuatnya lebih mudah digunakan.

Era Metaprogramming pra-ES6

Apakah Anda ingat eval? Mari kita lihat bagaimana itu digunakan:

const blog = { name: 'freeCodeCamp' } console.log('Before eval:', blog); const key = 'author'; const value = 'Tapas'; testEval = () => eval(`blog.${key} = '${value}'`); // Call the function testEval(); console.log('After eval magic:', blog); 

Seperti yang mungkin Anda perhatikan, bantuan evaldengan pembuatan kode tambahan. Dalam kasus ini, objek blogtelah dimodifikasi dengan properti tambahan pada waktu eksekusi.

Before eval: {name: freeCodeCamp} After eval magic: {name: "freeCodeCamp", author: "Tapas"} 

Introspeksi

Sebelum masuknya Reflect objectdi ES6, kita masih bisa melakukan introspeksi. Berikut adalah contoh pembacaan struktur program:

var users = { 'Tom': 32, 'Bill': 50, 'Sam': 65 }; Object.keys(users).forEach(name => { const age = users[name]; console.log(`User ${name} is ${age} years old!`); }); 

Di sini kita membaca usersstruktur objek dan mencatat nilai kunci dalam sebuah kalimat.

User Tom is 32 years old! User Bill is 50 years old! User Sam is 65 years old! 

Modifikasi Diri

Mari kita ambil objek blog yang memiliki metode untuk memodifikasi dirinya sendiri:

var blog = { name: 'freeCodeCamp', modifySelf: function(key, value) {blog[key] = value} } 

The blogobjek dapat memodifikasi sendiri dengan melakukan hal ini:

blog.modifySelf('author', 'Tapas'); 

Perantaraan

Intercessionadalah tentang bertindak atas nama hal lain dengan mengubah semantik bahasa. The   Object.defineProperty()Metode dapat mengubah semantik obyek:

var sun = {}; Object.defineProperty(sun, 'rises', { value: true, configurable: false, writable: false, enumerable: false }); console.log('sun rises', sun.rises); sun.rises = false; console.log('sun rises', sun.rises); 

Keluaran,

sun rises true sun rises true 

Seperti yang Anda lihat, sunobjek dibuat sebagai objek normal dan semantiknya telah diubah sehingga tidak dapat ditulis.

Sekarang mari kita mulai memahami objek Reflectdan Proxydengan penggunaannya masing-masing.

API Refleksi

Di ES6, Reflect adalah yang baru Global Object(seperti matematika) yang menyediakan sejumlah fungsi utilitas, banyak di antaranya tampak tumpang tindih dengan metode ES5 yang ditentukan secara global Object.

Semua fungsi ini adalah fungsi Introspeksi di mana Anda dapat menanyakan beberapa detail internal tentang program pada saat berjalan.

Berikut adalah daftar metode yang tersedia dari Reflectobjek. Silakan kunjungi halaman ini untuk melihat detail lebih lanjut dari masing-masing metode ini.

// Reflect object methods Reflect.apply() Reflect.construct() Reflect.get() Reflect.has() Reflect.ownKeys() Reflect.set() Reflect.setPrototypeOf() Reflect.defineProperty() Reflect.deleteProperty() Reflect.getOwnPropertyDescriptor() Reflect.getPrototypeOf() Reflect.isExtensible() 

But wait, here's a question: Why do we need a new API object when these could just exist already or could be added to Object or Function?

Confused? Let's try to figure this out.

All in one namespace

JavaScript already had support for object reflection. But these APIs were not organized under one namespace. Since ES6 they're now under Reflect.

Unlike most global objects, Reflect is not a constructor. You cannot use it with a new operator or invoke the Reflect object as a function. All properties and methods of Reflect are static like the math object.

Simple to use

The introspection methods of Object throw an exception when they fail to complete the operation. This is an added burden to the consumer (programmer) to handle that exception in the code.

You may prefer to handle it as a boolean(true | false) instead of using exception handling. The Reflect object helps you do that.

Here's an example with Object.defineProperty:

 try { Object.defineProperty(obj, name, desc); // property defined successfully } catch (e) { // possible failure and need to do something about it }

And with the Reflect API:

if (Reflect.defineProperty(obj, name, desc)) { // success } else { // failure (and far better) } 

The impression of the First-Class operation

We can find the existence of a property for an object as (prop in obj). If we need to use it multiple times in our code, we must explicitly wrap this operation in a function and pass the operation around as a first-class value.

In ES6, we already had those as part of the Reflect API as the first-class function. For example, Reflect.has(obj, prop) is the functional equivalent of (prop in obj).

Let's look at another example: Delete an object property.

const obj = { bar: true, baz: false}; // delete object[key] function deleteProperty(object, key) { delete object[key]; } deleteProperty(obj, 'bar'); 

With the Reflect API:

// With Reflect API Reflect.deleteProperty(obj, 'bar'); 

A more reliable way of using the apply() method

In ES5, we can use the apply() method to call a function with a given this value and passing an array as an argument.

Function.prototype.apply.call(func, obj, arr); // or func.apply(obj, arr); 

This is less reliable because func could be an object that would have defined its own apply method.

In ES6 we have a more reliable and elegant way of solving this:

Reflect.apply(func, obj, arr); 

In this case, we will get a TypeError if func is not callable. Also, Reflect.apply() is less verbose and easier to understand.

Helping other kinds of reflection

Wewill see what this means in a bit when we learn about the Proxy object. The Reflect API methods can be used with Proxy in many use cases.

The Proxy Object

ES6's Proxy object helps in intercession.

The proxy object defines custom behaviors for fundamental operations (for example, property lookup, assignment, enumeration, function invocation, and so on).

Here are a few useful terms you need to remember and use:

  • The target: An object which the proxy virtualizes.
  • The handler: A placeholder object which contains traps.
  • The trap: Methods that provide property access to the target object.

It is perfectly fine if you don't quite understand yet from the description above. We will get a grasp of it through code and examples in a minute.

The syntax to create a Proxy object is as follows:

let proxy = new Proxy(target, handler); 

There are many proxy traps (handler functions) available to access and customize a target object. Here is the list of them. You can read a more detailed description of traps here.

handler.apply() handler.construct() handler.get() handler.has() handler.ownKeys() handler.set() handler.setPrototypeOf() handler.getPrototypeOf() handler.defineProperty() handler.deleteProperty() handler.getOwnPropertyDescriptor() handler.preventExtensions() handler.isExtensible() 

Note that each of the traps has a mapping with the Reflect object's methods. This means that you can use Reflect and Proxy together in many use cases.

How to get unavailable object property values

Let's look at an example of an employee object and try to print some of its properties:

const employee = { firstName: 'Tapas', lastName: 'Adhikary' }; console.log(employee.firstName); console.log(employee.lastName); console.log(employee.org); console.log(employee.fullName); 

The expected output is the following:

Tapas Adhikary undefined undefined 

Now let's use the Proxy object to add some custom behavior to the employee object.

Step 1: Create a Handler that uses a get trap

We will use a trap called get which lets us get a property value. Here is our handler:

let handler = { get: function(target, fieldName) { if(fieldName === 'fullName' ) { return `${target.firstName} ${target.lastName}`; } return fieldName in target ? target[fieldName] : `No such property as, '${fieldName}'!` } }; 

The above handler helps to create the value for the fullName property. It also adds a better error message when an object property is missing.

Step 2: Create a Proxy Object

As we have the target employee object and the handler, we will be able to create a Proxy object like this:

let proxy = new Proxy(employee, handler); 

Step 3: Access the properties on the Proxy object

Now we can access the employee object properties using the proxy object, like this:

console.log(proxy.firstName); console.log(proxy.lastName); console.log(proxy.org); console.log(proxy.fullName); 

The output will be:

Tapas Adhikary No such property as, 'org'! Tapas Adhikary 

Notice how we have magically changed things for the employee object!

Proxy for Validation of Values

Let's create a proxy object to validate an integer value.

Step 1: Create a handler that uses a set trap

The handler looks like this:

const validator = { set: function(obj, prop, value) { if (prop === 'age') { if(!Number.isInteger(value)) { throw new TypeError('Age is always an Integer, Please Correct it!'); } if(value < 0) { throw new TypeError('This is insane, a negative age?'); } } } }; 

Step 2: Create a Proxy Object

Create a proxy object like this:

let proxy = new Proxy(employee, validator); 

Step 3: Assign a non-integer value to a property, say, age

Try doing this:

proxy.age = 'I am testing a blunder'; // string value 

The output will be like this:

TypeError: Age is always an Integer, Please Correct it! at Object.set (E:\Projects\KOSS\metaprogramming\js-mtprog\proxy\userSetProxy.js:28:23) at Object. (E:\Projects\KOSS\metaprogramming\js-mtprog\proxy\userSetProxy.js:40:7) at Module._compile (module.js:652:30) at Object.Module._extensions..js (module.js:663:10) at Module.load (module.js:565:32) at tryModuleLoad (module.js:505:12) at Function.Module._load (module.js:497:3) at Function.Module.runMain (module.js:693:10) at startup (bootstrap_node.js:188:16) at bootstrap_node.js:609:3 

Similarly, try doing this:

p.age = -1; // will result in error 

How to use Proxy and Reflect together

Here is an example of a handler where we use methods from the Reflect API:

const employee = { firstName: 'Tapas', lastName: 'Adhikary' }; let logHandler = { get: function(target, fieldName) { console.log("Log: ", target[fieldName]); // Use the get method of the Reflect object return Reflect.get(target, fieldName); } }; let func = () => { let p = new Proxy(employee, logHandler); p.firstName; p.lastName; }; func();

A few more Proxy use cases

There are several other use-cases where this concept can be used.

  • To protect the ID field of an object from deletion (trap: deleteProperty)
  • To trace Property Accesses (trap: get, set)
  • For Data Binding (trap: set)
  • With revocable references
  • To manipulate the in operator behavior

... and many more.

Metaprogramming Pitfalls

While the concept of Metaprogramming gives us lots of power, the magic of it can go the wrong way sometimes.

Be careful of:

  • Too much magic! Make sure you understand it before you apply it.
  • Possible performance hits when you're making the impossible possible
  • Could be seen as counter-debugging.

In Summary

To summarize,

  • Reflect and Proxy are great inclusions in JavaScript to help with Metaprogramming.
  • Lots of complex situations can be handled with their help.
  • Be aware of the downsides as well.
  • ES6 Symbols also can be used with your existing classes and objects to change their behavior.

I hope you found this article insightful. All the source code used in this article can be found in my GitHub repository.

Please share the article so others can read it as well. You can @ me on Twitter (@tapasadhikary) with comments, or feel free to follow me.