JavaScript Object.defineProperty() Method Example

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The static method Object.defineProperty() defines a new property directly on an object, or modifies an existing property on an object, and returns the object.

Example 1 - Creating a property

var user = {
    firstName : 'Ramesh',
    lastName : 'Fadatare',
    emailId : 'ramesh@gmail.com',
    age : 29
};

function test(){
    Object.defineProperty(user, 'fullName', {
        value: user.firstName + " " + user.lastName
      });
      console.log(user.fullName);
}

test();

Output:

Ramesh Fadatare

For the best learning experience, I highly recommended that you open a console (which, in Chrome and Firefox, can be done by pressing Ctrl+Shift+I), navigate to the "console" tab, copy-and-paste each JavaScript code example from this guide, and run it by pressing the Enter/Return key.

You can refer below screenshot for your testing:

Example 2 - Modifying a property

var o = {}; // Creates a new object

Object.defineProperty(o, 'a', {
  value: 37,
  writable: false
});

console.log(o.a); // logs 37
o.a = 25; // No error thrown
// (it would throw in strict mode,
// even if the value had been the same)
console.log(o.a); // logs 37. The assignment didn't work.

// strict mode
(function() {
  'use strict';
  var o = {};
  Object.defineProperty(o, 'b', {
    value: 2,
    writable: false
  });
  o.b = 3; // throws TypeError: "b" is read-only
  return o.b; // returns 2 without the line above
}());