Discussion

#123 F.prototype and constructor

easy
javascript

What will be output of following code:

function User(name) {
  this.name = name;
}

let user1 = new User('John');

console.log(user1.name); // ?

console.log(user1.constructor === User); // ?


User.prototype = {}; 

let user2 = new user1.constructor('Pete');

console.log( user2.name ); // ?

console.log(user12constructor === User) // ?

Output will be:

'John'
true
'Pete'
false

Explanation:

  1. user1.name:

    • user1 is created with the name 'John'.
    • Output: 'John'

  2. user1.constructor === User:

    • By default, an instance's constructor property points to the constructor function that created it.
    • Output: true

  3. User.prototype = {};:

    • This line changes the prototype of User to a new, empty object.
    • This affects new instances, not existing ones.

  4. user1.constructor('Pete'):

    • Despite the prototype change, user1.constructor still points to the original User function.
    • user2 is created with the name 'Pete'.
    • Output for user2.name: 'Pete'

  5. user2.constructor === User:

    • After the prototype change, new objects created from User have constructor set to Object, because the new prototype is an empty object {} which defaults to Object.
    • Output: false