JS: Reflect.defineProperty

By Xah Lee. Date: . Last updated: .

(new in ECMAScript 2015)

Reflect.defineProperty(obj, key, descriptor)

descriptor is Property Descriptor.

🟢 tip: This is a better version of Object.defineProperty. Because it returns success status.

// create a property that's not enumerable

const jj = {};

const result = Reflect.defineProperty(jj, "dog", {
 value: 3,
 writable: true,
 enumerable: false,
 configurable: true,
});

console.log(jj.dog);
// 3

console.log(
 Reflect.getOwnPropertyDescriptor(jj, "dog").enumerable ===
  false,
);
// true

// return value is boolean
console.log(result);
// true

Example. Make a property read-only

// make a property read-only
const jj = { dog: 3 };
Reflect.defineProperty(jj, "dog", { writable: false });

Note: once a property's “writable” attribute is set false, it can't be set back to true.

Example. Define Getter Property

const jj = {};

// add a getter property key xget
Reflect.defineProperty(
 jj,
 "xget",
 {
  get: function () {
   return "getter called";
  },
 },
);

console.assert(jj.xget === "getter called");

Example. Define Setter Property

const jj = {};

// add a setter property key xset
Reflect.defineProperty(
 jj,
 "xset",
 {
  set: function (x) {
   console.log("setter called with arg " + x);
  },
 },
);

jj.xset = 4;
// prints: setter called with arg 4