Skip to content
Related Articles

Related Articles

Improve Article
JavaScript | handler.defineProperty() Method
  • Last Updated : 15 Apr, 2020

The handler.defineProperty() method in Javascript is used to define the new properties and to modify the existing properties directly on an object. It is trap for Object.defineProper().


const p = new Proxy(target, {
  defineProperty: function(target, property, descriptor) {

Parameters: This method accepts three parameters as mentioned above and described below:

  • Target: This parameter holds the target object.
  • property: This parameter is the name or Symbol of the property whose description is going to be retrieved..
  • descriptor: This parameter is the descriptor for the property being defined or modified.

Return value: This method returns a Boolean value which is used to indicate if the property is successfully defined.

Below example illustrate the handler.defineProperty() method in JavaScript:

Example 1:

const p = new Proxy({}, {
  defineProperty: function(target, prop, descriptor) {
    console.log('Type : ' + prop);
    return true;
const desc = { configurable: true, enumerable: true, value: 10 };
Object.defineProperty(p, 'String', desc);
var xyz = {};  
var proxy = new Proxy(xyz, {  
  defineProperty: function(target, name, propertyDescriptor) {  
console.log('in defineProperty');  
return Object.defineProperty(target, name, propertyDescriptor);  
Object.defineProperty(proxy, 'bar', {} );  


"Type : String"
"in defineProperty"

Example 2:

const handler1 = {
  defineProperty(target, key, descriptor) {
    invariant(key, 'define');
    return true;
function invariant(key, action) {
  if (key[0] === '_') {
    throw new Error(
      `Invalid attempt to ${action} private "${key}" property`);
const monster1 = {};
const proxy1 = new Proxy(monster1, handler1);
console.log(proxy1._propt = 'Handler defineProperty');


Error: Invalid attempt to define private "_propt" property

Supported Browsers: The browsers supported by handler.defineProperty() method are listed below:

  • Google Chrome
  • Firefox
  • Opera
  • Safari
  • Edge

My Personal Notes arrow_drop_up
Recommended Articles
Page :