Hvad er metaprogrammering i JavaScript? På engelsk tak.

JavaScript har mange nyttige funktioner, som de fleste udviklere kender til. På samme tid er der nogle skjulte perler, der kan løse virkelig udfordrende problemer, hvis du er opmærksom på dem.

Metaprogrammering i JavaScript er et sådant koncept, som mange af os måske ikke er fortrolige med. I denne artikel vil vi lære om Metaprogramming, og hvordan det er nyttigt for os.

Med ES6 (ECMAScript 2015) har vi support til Reflectog Proxyobjekter, der giver os mulighed for let at udføre metaprogrammering. I denne artikel lærer vi, hvordan man bruger dem med eksempler.

Hvad er metaprogrammering?

Metaprogramminger intet mindre end magien i programmering ! Hvad med at skrive et program, der læser, ændrer, analyserer og endda genererer et program? Lyder det ikke magisk og kraftfuldt?

Sådan beskriver jeg Metaprogramming som en udvikler, der bruger det hele tiden:

Metaprogramminger en programmeringsteknik, hvor computerprogrammer har evnen til at behandle andre programmer som deres data. Dette betyder, at et program kan designes til at læse, generere, analysere eller transformere andre programmer og endda ændre sig selv, mens det kører.

Kort sagt, Metaprogramming indebærer at skrive kode, der kan

  • Generer kode
  • Manipulere sprogkonstruktioner på løbetiden. Dette fænomen er kendt som Reflective Metaprogrammingeller Reflection.

Hvad er refleksion i metaprogrammering?

Reflectioner en gren af ​​metaprogrammering. Refleksion har tre undergrene:

  1. Introspektion : Koden er i stand til at inspicere sig selv. Det bruges til at finde meget lave oplysninger om koden.
  2. Selvmodifikation : Som navnet antyder, kan kode ændre sig selv.
  3. Forbøn : Handler på vegne af en anden. Dette kan opnås ved at indpakke, fange, opfange.

ES6 giver os det Reflectobjekt (aka, Reflect API) at opnå Introspection. Den Proxygenstand ES6 hjælper os med Intercession. Vi vil ikke tale for meget om,   Self-Modificationda vi vil holde os væk fra det så meget som muligt.

Vent et øjeblik! Bare for at være klar blev Metaprogramming ikke introduceret i ES6. Det har snarere været tilgængeligt på sproget fra starten. ES6 gjorde det meget lettere at bruge.

Pre-ES6 æra af metaprogrammering

Kan du huske det eval? Lad os se på, hvordan det blev brugt:

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); 

Som du måske bemærker, evalhjalp det med yderligere kodegenerering. I dette tilfælde er objektet blogblevet ændret med en ekstra egenskab på udførelsestidspunktet.

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

Introspektion

Før inkluderingen af Reflect objecti ES6 kunne vi stadig foretage introspektion. Her er et eksempel på at læse programmets struktur:

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!`); }); 

Her læser vi usersobjektstrukturen og logger nøgleværdien i en sætning.

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

Selvmodifikation

Lad os tage et blogobjekt, der har en metode til at ændre sig selv:

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

Det blogobjekt kan ændre sig ved at gøre dette:

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

Forbøn

Intercessionhandler om at handle på vegne af noget andet ved at ændre sprogets semantik. Den   Object.defineProperty()metode kan ændre et objekts semantik:

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); 

Produktion,

sun rises true sun rises true 

Som du ser, blev sunobjektet oprettet som et normalt objekt, og derefter er semantikken ændret, så det ikke kan skrives.

Nu lad os hoppe ind i forståelsen af Reflectog Proxyobjekter med deres respektive kutymer.

Reflect API

I ES6 er Reflect en ny Global Object(som matematik), der giver en række hjælpefunktioner, hvoraf mange synes at overlappe med ES5-metoder, der er defineret på det globale Object.

Alle disse funktioner er Introspection-funktioner, hvor du kan spørge nogle interne detaljer om programmet på kørselstidspunktet.

Her er listen over tilgængelige metoder fra Reflectobjektet. Besøg denne side for at se flere detaljer om hver af disse metoder.

// 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.
  • Vær også opmærksom på ulemperne.
  • ES6-symboler kan også bruges sammen med dine eksisterende klasser og objekter til at ændre deres adfærd.

Jeg håber, du fandt denne artikel indsigtsfuld. Al kildekoden, der bruges i denne artikel, kan findes i mit GitHub-lager.

Del artiklen, så andre også kan læse den. Du kan @ mig på Twitter (@tapasadhikary) med kommentarer eller være velkommen til at følge mig.