Meta programming
Proxies
Proxy objects allow you to intercept certain operations and to implement custom behaviors.For example, getting a property on an object:
js
const handler = {
get(target, name) {
return name in target ? target[name] : 42;
},
};
const p = new Proxy({}, handler);
p.a = 1;
console.log(p.a, p.b); // 1, 42
The
Proxy object defines a target (an empty 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
TypeErrorwill 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 |
|
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() |
|
handler.defineProperty() |
Object.defineProperty()Reflect.defineProperty() |
|
handler.has() |
|
|
handler.get() |
|
|
handler.set() |
|
|
handler.deleteProperty() |
|
A property cannot be deleted if it exists as a non-configurable own
property of target.
|
handler.ownKeys() |
Object.getOwnPropertyNames()Object.getOwnPropertySymbols()Object.keys()Reflect.ownKeys() |
|
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 on the proxy leads to a
TypeError.js
const revocable = Proxy.revocable(
{},
{
get(target, name) {
return `[[${name}]]`;
},
},
);
const proxy = revocable.proxy;
console.log(proxy.foo); // "[[foo]]"
revocable.revoke();
console.log(proxy.foo); // TypeError: Cannot perform 'get' on a proxy that has been revoked
proxy.foo = 1; // TypeError: Cannot perform 'set' on a proxy that has been revoked
delete proxy.foo; // TypeError: Cannot perform 'deleteProperty' on a proxy that has been revoked
console.log(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:js
Reflect.has(Object, "assign"); // true
A better apply() function
Before
Reflect, 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).js
Function.prototype.apply.call(Math.floor, undefined, [1.75]);
With
Reflect.apply this becomes less verbose and easier to understand:js
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:js
if (Reflect.defineProperty(target, property, attributes)) {
// success
} else {
// failure
}