Reflect.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 Reflect.set() static method is like the property accessor and assignment syntax, but as a function.

Try it

const object = {};
Reflect.set(object, "foo", 42);

console.log(object.foo);
// Expected output: 42

const array = ["duck", "duck", "duck"];
Reflect.set(array, 2, "goose");

console.log(array[2]);
// Expected output: "goose"

Syntax

js
Reflect.set(target, propertyKey, value)
Reflect.set(target, propertyKey, value, receiver)

Parameters

target

The target object on which to set the property.

propertyKey

The name of the property to set.

value

The value to set.

receiver Optional

The value of this provided for the call to the setter for propertyKey on target. If provided and target does not have a setter for propertyKey, the property will be set on receiver instead. Defaults to target.

Return value

A Boolean indicating whether or not setting the property was successful.

Exceptions

TypeError

Thrown if target is not an object.

Description

Reflect.set() provides the reflective semantic of a property access. That is, Reflect.set(target, propertyKey, value, receiver) is semantically equivalent to:

target[propertyKey] = value;

Note that in a normal property access, target and receiver would observably be the same object.

Reflect.set() invokes the [[Set]] object internal method of target.

Examples

Using Reflect.set()

js
// Object
const obj = {};
Reflect.set(obj, "prop", "value"); // true
obj.prop; // "value"

// Array
const arr = ["duck", "duck", "duck"];
Reflect.set(arr, 2, "goose"); // true
arr[2]; // "goose"

// It can truncate an array.
Reflect.set(arr, "length", 1); // true
arr; // ["duck"]

// With just one argument, propertyKey and value are "undefined".
Reflect.set(obj); // true
Reflect.getOwnPropertyDescriptor(obj, "undefined");
// { value: undefined, writable: true, enumerable: true, configurable: true }

Different target and receiver

When the target and receiver are different, Reflect.set will use the property descriptor of target (to find the setter or determine if the property is writable), but set the property on receiver.

js
const target = {};
const receiver = {};
Reflect.set(target, "a", 2, receiver); // true
// target is {}; receiver is { a: 2 }

const target = { a: 1 };
const receiver = {};
Reflect.set(target, "a", 2, receiver); // true
// target is { a: 1 }; receiver is { a: 2 }

const target = {
  set a(v) {
    this.b = v;
  },
};
const receiver = {};
Reflect.set(target, "a", 2, receiver); // true
// target is { a: [Setter] }; receiver is { b: 2 }