handler.set()

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since September 2016.

The handler.set() method is a trap for the [[Set]] object internal method, which is used by operations such as using property accessors to set a property's value.

Try it

const monster1 = { eyeCount: 4 };

const handler1 = {
  set(obj, prop, value) {
    if (prop === "eyeCount" && value % 2 !== 0) {
      console.log("Monsters must have an even number of eyes");
    } else {
      return Reflect.set(...arguments);
    }
  },
};

const proxy1 = new Proxy(monster1, handler1);

proxy1.eyeCount = 1;
// Expected output: "Monsters must have an even number of eyes"

console.log(proxy1.eyeCount);
// Expected output: 4

proxy1.eyeCount = 2;
console.log(proxy1.eyeCount);
// Expected output: 2

Syntax

js
new Proxy(target, {
  set(target, property, value, receiver) {
  }
})

Parameters

The following parameters are passed to the set() method. this is bound to the handler.

target

The target object.

property

A string or Symbol representing the property name.

value

The new value of the property to set.

receiver

The this value for setters; see Reflect.set(). This is usually either the proxy itself or an object that inherits from the proxy.

Return value

The set() method must return a Boolean indicating whether or not the assignment succeeded. Other values are coerced to booleans.

Many operations, including using property accessors in strict mode, throw a TypeError if the [[Set]] internal method returns false.

Description

Interceptions

This trap can intercept these operations:

  • Property assignment: proxy[foo] = bar and proxy.foo = bar
  • Reflect.set()

Or any other operation that invokes the [[Set]] internal method.

Invariants

The proxy's [[Set]] internal method throws a TypeError if the handler definition violates one of the following invariants:

  • Cannot change the value of a property to be different from the value of the corresponding target object property, if the corresponding target object property is a non-writable, non-configurable own data property. That is, if Reflect.getOwnPropertyDescriptor() returns configurable: false, writable: false for the property on target, and value is different from the value attribute in the target's property descriptor, then the trap must return a falsy value.
  • Cannot set the value of a property if the corresponding target object property is a non-configurable own accessor property that has an undefined setter. That is, if Reflect.getOwnPropertyDescriptor() returns configurable: false, set: undefined for the property on target, then the trap must return a falsy value.

Examples

Trap setting of a property value

The following code traps setting a property value.

js
const p = new Proxy(
  {},
  {
    set(target, prop, value, receiver) {
      target[prop] = value;
      console.log(`property set: ${prop} = ${value}`);
      return true;
    },
  },
);

console.log("a" in p); // false

p.a = 10; // "property set: a = 10"
console.log("a" in p); // true
console.log(p.a); // 10

Specifications

Specification
ECMAScript® 2025 Language Specification
# sec-proxy-object-internal-methods-and-internal-slots-set-p-v-receiver

Browser compatibility

Report problems with this compatibility data on GitHub
desktopmobileserver
Chrome
Edge
Firefox
Opera
Safari
Chrome Android
Firefox for Android
Opera Android
Safari on iOS
Samsung Internet
WebView Android
WebView on iOS
Deno
Node.js
set

Legend

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Full support
Full support

See also