@rbxts/jsnatives

Proxy - set hook

The set hook is triggered when a property is set on the proxy.

Signature

function set<T>(target: T, key: unknown, value: unknown, proxy: T): boolean;

Description

The set handler trap is called when a property is set on the proxy object. It allows you to intercept and customize property assignment operations.

Parameters

target: The original target object the proxy is wrapping
key: The property key (string, number, or symbol) being set
value: The new value being assigned to the property
proxy: The proxy instance itself

Return value

A boolean indicating whether the assignment succeeded. Return true to indicate success, or false to indicate failure (which will trigger an error).

Examples

Basic property assignment interception

const target = {};
const proxy = new Proxy(target, {
  set: (target, prop, value, proxy) => {
    console.log(`Setting property ${String(prop)} to ${value}`);
    target[prop] = value;
    return true; // Indicate success
  }
});

proxy.name = "John"; // Logs: "Setting property name to John"
console.log(target.name); // "John" (the change is reflected in the target)

Validation

const numbers = [];
const proxy = new Proxy(numbers, {
  set: (target, prop, value, proxy) => {
    // Ensure we're only adding numbers to the array
    if (typeIs(value, "number")) {
      target[prop] = value;
      return true;
    }
    console.error("Only numbers can be added to this array");
    return false; // Reject the operation
  }
});

proxy.push(1); // Works
proxy.push(2); // Works
// proxy.push("3"); // Logs: "Only numbers can be added to this array" and fails with error "[PROXYERROR] Failed to set value on target object"
console.log(numbers); // [1, 2]

Transformation

const target = {};
const proxy = new Proxy(target, {
  set: (target, prop, value, proxy) => {
    // Convert all string values to uppercase
    if (typeof value === "string") {
      target[prop] = value.toUpperCase();
    } else {
      target[prop] = value;
    }
    return true;
  }
});

proxy.name = "john";
proxy.age = 30;
console.log(target); // { name: "JOHN", age: 30 }

Using raw properties to bypass the set trap

const target = {};
const raw = { directProp: "unchanged" };
const proxy = new Proxy(target, {
  set: (target, prop, value, proxy) => {
    console.log(`Set trap for ${String(prop)}`);
    target[prop] = `Modified: ${value}`;
    return true;
  }
}, raw);

proxy.normalProp = "value"; // Logs: "Set trap for normalProp"
proxy.directProp = "changed"; // Doesn't trigger the set trap
console.log(proxy.normalProp); // "Modified: value" (went through set trap)
console.log(proxy.directProp); // "changed" (raw property was modified directly)