JavaScript ima mnogo korisnih značajki o kojima većina programera zna. Istodobno, postoje neki skriveni dragulji koji mogu riješiti doista izazovne probleme ako ste ih svjesni.

Metaprogramiranje u JavaScript-u jedan je od takvih koncepata koji mnogima od nas možda nije poznat. U ovom ćemo članku naučiti o metaprogramiranju i kako nam je korisno.

Uz ES6 (ECMAScript 2015) imamo podršku za Reflecti Proxyobjekte koji nam omogućuju s lakoćom metaprogramiranje. U ovom ćemo članku naučiti kako ih koristiti s primjerima.

Što je metaprogramiranje?

Metaprogrammingnije ništa manje od čarolije u programiranju ! Što kažete na pisanje programa koji čita, mijenja, analizira, pa čak i generira program? Ne zvuči li to čarobno i moćno?

Evo kako bih Metaprogramiranje opisao kao programera koji ga stalno koristi:

Metaprogrammingje tehnika programiranja u kojoj računalni programi imaju mogućnost tretirati druge programe kao svoje podatke. To znači da program može biti dizajniran za čitanje, generiranje, analizu ili transformiranje drugih programa, pa čak i da se sam mijenja tijekom izvođenja.

Jednostavno rečeno, metaprogramiranje uključuje pisanje koda koji može

Generiraj kod
Manipuliranje jezičnim konstrukcijama u vrijeme izvođenja. Taj je fenomen poznat kao Reflective Metaprogrammingili Reflection.

Što je refleksija u metaprogramiranju?

Reflectionje grana Metaprogramiranja. Reflection ima tri podgrane:

Introspekcija : Kôd je u stanju sam se pregledati. Koristi se za otkrivanje vrlo niskih podataka o kodu.
Samo-modifikacija : Kao što samo ime govori, kod se može sam mijenjati.
Zagovor : Djelujući u ime nekoga drugoga. To se može postići omatanjem, hvatanjem, presretanjem.

ES6 nam daje Reflectcilj (odnosno Reflect API) koji moramo postići Introspection. ProxyPredmet ES6 nam pomaže Intercession. Nećemo previše razgovarati o tome Self-Modificationjer se želimo što više kloniti toga.

Samo trenutak! Da budemo jasni, metaprogramiranje nije uvedeno u ES6. Umjesto toga, dostupan je na jeziku od samog početka. ES6 je samo puno olakšao upotrebu.

Pre-ES6 era metaprogramiranja

Sjećate li se eval? Pogledajmo kako je korišten:

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);

Kao što ste primijetili, evalpomogao je s dodatnim generiranjem koda. U ovom je slučaju objekt blogizmijenjen dodatnim svojstvom u vrijeme izvršenja.

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

Introspekcija

Prije uključivanja Reflect objectES-a u ES6 još smo uvijek mogli obaviti introspekciju. Evo primjera čitanja strukture programa:

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!`); });

Ovdje čitamo usersstrukturu objekta i bilježimo ključ / vrijednost u rečenici.

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

Samoizmjena

Uzmimo objekt bloga koji ima metodu za samoizmjenu:

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

blogObjekt može se mijenjati na taj način:

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

Zagovor

Intercessionradi o djelovanju u ime nečeg drugog mijenjanjem semantike jezika. Object.defineProperty()Metoda može promijeniti neki objekt u semantiku:

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);

Izlaz,

sun rises true sun rises true

Kao što vidite, sunobjekt je stvoren kao normalan objekt, a zatim je semantika promijenjena tako da se na njega ne može pisati.

Sada krenimo u razumijevanje Reflecti Proxyobjekata s njihovim odgovarajućim načinima korištenja.

API Reflect

U ES6, Reflect je novost Global Object(poput matematike) koja pruža brojne korisne funkcije, od kojih se čini da se mnoge preklapaju s ES5 metodama definiranim na globalnoj razini Object.

Sve ove funkcije su funkcije introspekcije u kojima možete zatražiti neke interne detalje o programu u vrijeme izvođenja.

Ovdje je popis dostupnih metoda iz Reflectobjekta. Posjetite ovu stranicu da biste vidjeli više detalja o svakoj od ovih metoda.

// 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.