Proxy forwarding
A proxy logs, validates, or measures property reads and writes before delegating normal behavior.
JS Puzzles · Const
Reflect methods
Reflect exposes function-based forms of interceptable object operations, with method names matching Proxy traps. It is commonly used in proxy handlers to forward the default operation after adding custom behavior.
How it works
Reflect provides function forms of JavaScript's interceptable object operations. Its methods correspond to Proxy traps, so a proxy handler can add behavior and then delegate the default operation to Reflect with the same arguments.
Syntax
Reflect.method(target, ...arguments)Returns
Individual Reflect methods return the result of their corresponding object operation; for example, Reflect.get returns a property value and Reflect.set returns a success boolean.
Key points
- Reflect is especially useful inside proxy traps: it lets custom behavior delegate the underlying operation without manually reproducing it.
- Reflect's static method names align with Proxy trap names, making the API predictable when forwarding operations.
- Reflect.get(target, key) performs a property lookup and returns the resulting value.
- Reflect.set(target, key, value[, receiver]) performs assignment and returns whether the update succeeded.
- Reflect.construct(target, argumentsList) invokes a constructor with a dynamically supplied argument list.
Common mistakes
- Reflect.set() reports assignment success with a boolean instead of exposing assignment syntax's result.
- Reflect does not replace Proxy; it supplies the corresponding reflective operation used to forward a trap.
- Reflect methods closely mirror low-level object operations, so they still perform the operation on the supplied target.
Example: Forward proxy operations with Reflect
const target = { value: 1 };
const proxy = new Proxy(target, {
get(object, property, receiver) {
console.log(`read: ${String(property)}`);
return Reflect.get(object, property, receiver);
},
set(object, property, value, receiver) {
console.log(`write: ${String(property)}`);
return Reflect.set(object, property, value, receiver);
}
});
console.log(proxy.value);
proxy.value = 2;
console.log(target.value);The proxy logs operations, while Reflect forwards each operation to the target using the trap's matching arguments.
When to use it
Dynamic object operations
Code needs property access, assignment, deletion, or construction through dynamically selected operations.
Default semantic delegation
A proxy handler must preserve ordinary JavaScript semantics instead of reimplementing inheritance and accessors.
Interview questions
QWhat is Reflect used for?
Reflect is a built-in object whose static methods invoke interceptable object internal methods. Each method corresponds to a Proxy handler method, such as Reflect.get for property reads and Reflect.set for writes.
Follow-up: How can a proxy preserve normal behavior after logging a property read? The get trap should call Reflect.get with the target, property key, and receiver.
QHow do Reflect.get() and Reflect.set() map to ordinary operations?
Reflect.get(target, key) performs a property read as a function call, while Reflect.set(target, key, value) performs an assignment-like operation. Reflect.set returns a boolean indicating whether the update succeeded.
Follow-up: Why pass receiver to Reflect.get or Reflect.set? It preserves the receiver used by accessor lookup and assignment, which matters when inherited getters or setters are involved.
QWhat does Reflect.construct() do?
Reflect.construct(target, argumentsList) performs the constructor operation represented by new target(...argumentsList). Reflect.construct can also receive newTarget, allowing the constructed object's prototype relationship to use a different constructor.
Follow-up: What does newTarget change? It changes the constructor's new.target value and can make the resulting object an instance of newTarget rather than target.
Related
ProxyReflect.getReflect.setReflect.deletePropertyReflect.constructObject internal methods