files/en-us/web/javascript/guide/meta_programming/index.md
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.
{{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.
The following terms are used when talking about the functionality of proxies.
The following table summarizes the available traps available to Proxy objects. See the reference pages for detailed explanations and examples.
ProxyThe {{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 {{jsxref("TypeError")}}.
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console.log(proxy.foo); // TypeError: Cannot perform 'get' on a proxy that has been revoked
proxy.foo = 1; // TypeError: Cannot perform 'set' on a proxy that has been revoked
delete proxy.foo; // TypeError: Cannot perform 'deleteProperty' on a proxy that has been revoked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
{{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
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"
With {{jsxref("Object.defineProperty")}}, which returns an object if successful, or throws a {{jsxref("TypeError")}} 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
}