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Meta programming

Starting with ECMAScript 6, JavaScript gains support for the Proxy and Reflect objects allowing you to intercept and define custom behavior for fundamental language operations (e.g. property lookup, assignment, enumeration, function invocation, etc). With the help of these two objects you are able to program at the meta level of JavaScript.

Proxies

Introduced in ECMAScript 6, Proxy objects allow you to intercept certain operations and to implement custom behaviors. For example getting a property on an object:

var handler = {
  get: function(target, name){
    return name in target ? target[name] : 42;
}};
var p = new Proxy({}, handler);
p.a = 1;
console.log(p.a, p.b); // 1, 42

The Proxy object defines a target (an object here) and a handler object in which a get trap is implemented. Here, an object that is proxied will not return undefined when getting undefined properties, but will instead return the number 42.

Additional examples are available on the Proxy reference page.

Terminology

The following terms are used when talking about the functionality of proxies.

handler
Placeholder object which contains traps.
traps
The methods that provide property access. This is analogous to the concept of traps in operating systems.
target
Object which the proxy virtualizes. It is often used as storage backend for the proxy. Invariants (semantics that remain unchanged) regarding object non-extensibility or non-configurable properties are verified against the target.
invariants
Semantics that remain unchanged when implementing custom operations are called invariants. If you violate the invariants of a handler, a TypeError will be thrown.

Handlers and traps

The following table summarizes the available traps available to Proxy objects. See the reference pages for detailed explanations and examples.

Handler / trap Interceptions Invariants
handler.getPrototypeOf() Object.getPrototypeOf()
Reflect.getPrototypeOf()
__proto__
Object.prototype.isPrototypeOf()
instanceof
  • getPrototypeOf method must return an object or null.
  • If target is not extensible, Object.getPrototypeOf(proxy) method must return the same value as Object.getPrototypeOf(target).
handler.setPrototypeOf() Object.setPrototypeOf()
Reflect.setPrototypeOf()
If target is not extensible, the prototype parameter must be the same value as Object.getPrototypeOf(target).
handler.isExtensible() Object.isExtensible()
Reflect.isExtensible()
Object.isExtensible(proxy) must return the same value as Object.isExtensible(target).
handler.preventExtensions() Object.preventExtensions()
Reflect.preventExtensions()
Object.preventExtensions(proxy) only returns true if Object.isExtensible(proxy) is false.
handler.getOwnPropertyDescriptor() Object.getOwnPropertyDescriptor()
Reflect.getOwnPropertyDescriptor()
  • getOwnPropertyDescriptor must return an object or undefined.
  • A property cannot be reported as non-existent, if it exists as a non-configurable own property of the target object.
  • A property cannot be reported as non-existent, if it exists as an own property of the target object and the target object is not extensible.
  • A property cannot be reported as existent, if it does not exists as an own property of the target object and the target object is not extensible.
  • A property cannot be reported as non-configurable, if it does not exists as an own property of the target object or if it exists as a configurable own property of the target object.
  • The result of Object.getOwnPropertyDescriptor(target) can be applied to the target object using Object.defineProperty and will not throw an exception.
handler.defineProperty() Object.defineProperty()
Reflect.defineProperty()
  • A property cannot be added, if the target object is not extensible.
  • A property cannot be added as or modified to be non-configurable, if it does not exists as a non-configurable own property of the target object.
  • A property may not be non-configurable, if a corresponding configurable property of the target object exists.
  • If a property has a corresponding target object property then Object.defineProperty(target, prop, descriptor) will not throw an exception.
  • In strict mode, a false return value from the defineProperty handler will throw a TypeError exception.
handler.has() Property query: foo in proxy
Inherited property query: foo in Object.create(proxy)
Reflect.has()
  • A property cannot be reported as non-existent, if it exists as a non-configurable own property of the target object.
  • A property cannot be reported as non-existent, if it exists as an own property of the target object and the target object is not extensible.
handler.get() Property access: proxy[foo]and proxy.bar
Inherited property access: Object.create(proxy)[foo]
Reflect.get()
  • The value reported for a property must be the same as the value of the corresponding target object property if the target object property is a non-writable, non-configurable data property.
  • The value reported for a property must be undefined if the corresponding target object property is non-configurable accessor property that has undefined as its [[Get]] attribute.
handler.set() Property assignment: proxy[foo] = bar and proxy.foo = bar
Inherited property assignment: Object.create(proxy)[foo] = bar
Reflect.set()
  • Cannot change the value of a property to be different from the value of the corresponding target object property if the corresponding target object property is a non-writable, non-configurable data property.
  • Cannot set the value of a property if the corresponding target object property is a non-configurable accessor property that has undefined as its [[Set]] attribute.
  • In strict mode, a false return value from the set handler will throw a TypeError exception.
handler.deleteProperty() Property deletion: delete proxy[foo] and delete proxy.foo
Reflect.deleteProperty()
A property cannot be deleted, if it exists as a non-configurable own property of the target object.
handler.enumerate() Property enumeration / for...in: for (var name in proxy) {...}
Reflect.enumerate()
The enumerate method must return an object.
handler.ownKeys() Object.getOwnPropertyNames()
Object.getOwnPropertySymbols()
Object.keys()
Reflect.ownKeys()
  • The result of ownKeys is a List.
  • The Type of each result List element is either String or Symbol.
  • The result List must contain the keys of all non-configurable own properties of the target object.
  • If the target object is not extensible, then the result List must contain all the keys of the own properties of the target object and no other values.
handler.apply() proxy(..args)
Function.prototype.apply() and Function.prototype.call()
Reflect.apply()
There are no invariants for the handler.apply method.
handler.construct() new proxy(...args)
Reflect.construct()
The result must be an Object.

Revocable Proxy

The Proxy.revocable() method is used to create a revocable Proxy object. This means that the proxy can be revoked via the function revoke and switches the proxy off. Afterwards, any operation leads on the proxy leads to a TypeError.

var revocable = Proxy.revocable({}, {
  get: function(target, name) {
    return "[[" + name + "]]";
  }
});
var proxy = revocable.proxy;
console.log(proxy.foo); // "[[foo]]"

revocable.revoke();

console.log(proxy.foo); // TypeError is thrown
proxy.foo = 1           // TypeError again
delete proxy.foo;       // still TypeError
typeof proxy            // "object", typeof doesn't trigger any trap

Reflection

Reflect is a built-in object that provides methods for interceptable JavaScript operations. The methods are the same as those of the proxy handlers. Reflect is not a function object.

Reflect helps with forwarding default operations from the handler to the target.

With Reflect.has() for example, you get the in operator as a function:

Reflect.has(Object, "assign"); // true

A better apply function

In ES5, you typically use the Function.prototype.apply() method to call a function with a given this value and arguments provided as an array (or an array-like object).

Function.prototype.apply.call(Math.floor, undefined, [1.75]);

With Reflect.apply this becomes less verbose and easier to understand:

Reflect.apply(Math.floor, undefined, [1.75]); 
// 1;

Reflect.apply(String.fromCharCode, undefined, [104, 101, 108, 108, 111]);
// "hello"

Reflect.apply(RegExp.prototype.exec, /ab/, ["confabulation"]).index;
// 4

Reflect.apply("".charAt, "ponies", [3]);
// "i"

Checking if property definition has been successful

With Object.defineProperty, which returns an object if successful, or throws a TypeError otherwise, you would use a try...catch block to catch any error that occurred while defining a property. Because Reflect.defineProperty returns a Boolean success status, you can just use an if...else block here:

if (Reflect.defineProperty(target, property, attributes)) {
  // success
} else {
  // failure
}

Document Tags and Contributors

 Contributors to this page: x2357, fscholz, bhtp, SphinxKnight, andres5099
 Last updated by: x2357,