JS Meta & Proxy

Meta programming enables dynamic behavior at runtime. Understand Proxy and Reflect for advanced object control.

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Meta Programming

Meta programming means writing code that can inspect or modify the behavior of other code at runtime. In JavaScript, this is mainly done using Reflect and Proxy.

Common meta-programming use cases:

Validation layers
Logging and debugging
Access control
Reactive frameworks (like Vue)

Meta Reflect

The Reflect object provides methods that mirror many internal object operations. It offers a cleaner and more consistent API than older Object methods.

const user = { name: "Ozan" };

Reflect.set(user, "age", 30);
console.log(Reflect.get(user, "name"));

console.log(Reflect.has(user, "age"));
Reflect.deleteProperty(user, "age");

Reflect methods return booleans instead of throwing errors in some cases, which makes them safer in advanced logic.

Meta Proxy

A Proxy wraps an object and intercepts operations like reading, writing, deleting, or calling functions.

const target = { name: "Jane" };

const handler = {
  get(obj, prop) {
    console.log("Getting property:", prop);
    return obj[prop];
  },
  set(obj, prop, value) {
    console.log("Setting", prop, "to", value);
    obj[prop] = value;
    return true;
  }
};

const proxy = new Proxy(target, handler);

console.log(proxy.name);
proxy.age = 25;

Common traps:

get
set
has
deleteProperty
apply (for functions)
construct (for constructors)

Validation Example

const person = {
  name: "Ali",
  age: 20
};

const validated = new Proxy(person, {
  set(obj, prop, value) {
    if (prop === "age" && typeof value !== "number") {
      throw new TypeError("Age must be a number");
    }
    obj[prop] = value;
    return true;
  }
});

validated.age = 30;
// validated.age = "thirty"; // throws error

Function Proxy

function sum(a, b) {
  return a + b;
}

const proxiedSum = new Proxy(sum, {
  apply(target, thisArg, args) {
    console.log("Called with:", args);
    return Reflect.apply(target, thisArg, args);
  }
});

console.log(proxiedSum(2, 3));