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Meta programming

The {{jsxref("Proxy")}} and {{jsxref("Reflect")}} objects allow 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​

{{jsxref("Proxy")}} objects allow you to intercept certain operations and to implement custom behaviors.

For example, getting a property on an object:

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 {{jsxref("Proxy")}} reference page.

Terminology​

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

  • {{jsxref("Proxy/Proxy", "handler", "", 1)}}
    • : 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 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
{{jsxref("Proxy/Proxy/getPrototypeOf", "handler.getPrototypeOf()")}} {{jsxref("Object.getPrototypeOf()")}}
{{jsxref("Reflect.getPrototypeOf()")}}
{{jsxref("Object/proto", "__proto__")}}
{{jsxref("Object.prototype.isPrototypeOf()")}}
{{jsxref("Operators/instanceof", "instanceof")}}
{{jsxref("Proxy/Proxy/setPrototypeOf", "handler.setPrototypeOf()")}} {{jsxref("Object.setPrototypeOf()")}}
{{jsxref("Reflect.setPrototypeOf()")}}
{{jsxref("Proxy/Proxy/isExtensible", "handler.isExtensible()")}} {{jsxref("Object.isExtensible()")}}
{{jsxref("Reflect.isExtensible()")}}
{{jsxref("Proxy/Proxy/preventExtensions", "handler.preventExtensions()")}} {{jsxref("Object.preventExtensions()")}}
{{jsxref("Reflect.preventExtensions()")}}
{{jsxref("Proxy/Proxy/getOwnPropertyDescriptor", "handler.getOwnPropertyDescriptor()")}} {{jsxref("Object.getOwnPropertyDescriptor()")}}
{{jsxref("Reflect.getOwnPropertyDescriptor()")}}
{{jsxref("Proxy/Proxy/defineProperty", "handler.defineProperty()")}} {{jsxref("Object.defineProperty()")}}
{{jsxref("Reflect.defineProperty()")}}
{{jsxref("Proxy/Proxy/has", "handler.has()")}}
Property query
foo in proxy
Inherited property query
foo in Object.create(proxy)

{{jsxref("Reflect.has()")}}
{{jsxref("Proxy/Proxy/get", "handler.get()")}}
Property access
proxy[foo]

proxy.bar
Inherited property access
Object.create(proxy)[foo]

{{jsxref("Reflect.get()")}}
{{jsxref("Proxy/Proxy/set", "handler.set()")}}
Property assignment
proxy[foo] = bar

proxy.foo = bar
Inherited property assignment
Object.create(proxy)[foo] = bar

{{jsxref("Reflect.set()")}}
{{jsxref("Proxy/Proxy/deleteProperty", "handler.deleteProperty()")}}
Property deletion
delete proxy[foo]

delete proxy.foo

{{jsxref("Reflect.deleteProperty()")}}
{{jsxref("Proxy/Proxy/ownKeys", "handler.ownKeys()")}} {{jsxref("Object.getOwnPropertyNames()")}}
{{jsxref("Object.getOwnPropertySymbols()")}}
{{jsxref("Object.keys()")}}
{{jsxref("Reflect.ownKeys()")}}
{{jsxref("Proxy/Proxy/apply", "handler.apply()")}} proxy(..args)
{{jsxref("Function.prototype.apply()")}} and {{jsxref("Function.prototype.call()")}}
{{jsxref("Reflect.apply()")}}
{{jsxref("Proxy/Proxy/construct", "handler.construct()")}} new proxy(...args)
{{jsxref("Reflect.construct()")}}

Revocable Proxy​

The {{jsxref("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 .

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​

{{jsxref("Reflect")}} is a built-in object that provides methods for interceptable JavaScript operations. The methods are the same as those of the proxy handler's.

Reflect is not a function object.

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

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

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

A better apply() function​

Before Reflect, you typically use the {{jsxref("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 {{jsxref("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 {{jsxref("Object.defineProperty")}}, which returns an object if successful, or throws a otherwise, you would use a {{jsxref("Statements/try...catch", "try...catch")}} block to catch any error that occurred while defining a property. Because {{jsxref("Reflect.defineProperty()")}} returns a Boolean success status, you can just use an {{jsxref("Statements/if...else", "if...else")}} block here:

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

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