Strange behavior of Object.defineProperty() in JavaScript

You should set enumerable to true. In Object.defineProperty its false by default. According to MDN.

enumerable

true if and only if this property shows up during enumeration of the properties on the corresponding object.

Defaults to false.

Non-enumerable means that property will not be shown in Object.keys() or for..in loop neither in console

let profile = {
    name: 'Barry Allen',
}

// I added a new property in the profile object.

Object.defineProperty(profile , 'age', {
    value: 23,
    writable: true,
    enumerable: true
})
console.log(profile)
console.log(profile.age)

All the properties and methods on prototype object of built-in classes are non-enumerable. Thats is the reason you can call them from instance but they don't appear while iterating.

To get all properties(including non-enumerable)Object​.get​OwnProperty​Names() .

let profile = {
    name: 'Barry Allen',
}

// I added a new property in the profile object.

Object.defineProperty(profile , 'age', {
    value: 23,
    writable: true,
    enumerable: false
})
for(let key in profile) console.log(key) //only name will be displayed.

console.log(Object.getOwnPropertyNames(profile)) //You will se age too

By default, properties you define with defineProperty are not enumerable - this means that they will not show up when you iterate over their Object.keys (which is what the snippet console does). (Similarly, the length property of an array does not get displayed, because it's non-enumerable.)

See MDN:

enumerable

true if and only if this property shows up during enumeration of the properties on the corresponding object.

Defaults to false.

Make it enumerable instead:

//Code Snippet 
let profile = {
  name: 'Barry Allen',
}

// I added a new property in the profile object.
Object.defineProperty(profile, 'age', {
  value: 23,
  writable: true,
  enumerable: true
})

console.log(profile)
console.log(profile.age)

The reason you can see the property in the logged image is that Chrome's console will show you non-enumerable properties as well - but the non-enumerable properties will be slightly greyed-out:

See how age is grey-ish, while name is not - this indicates that name is enumerable, and age is not.