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Changing "prototype"

importance: 5

In the code below we create new Rabbit, and then try to modify its prototype.

In the start, we have this code:

function Rabbit() {}
Rabbit.prototype = {
  eats: true
};

let rabbit = new Rabbit();

alert( rabbit.eats ); // true
  1. We added one more string (emphasized), what alert shows now?

    function Rabbit() {}
    Rabbit.prototype = {
      eats: true
    };
    
    let rabbit = new Rabbit();
    
    Rabbit.prototype = {};
    
    alert( rabbit.eats ); // ?
  2. …And if the code is like this (replaced one line)?

    function Rabbit() {}
    Rabbit.prototype = {
      eats: true
    };
    
    let rabbit = new Rabbit();
    
    Rabbit.prototype.eats = false;
    
    alert( rabbit.eats ); // ?
  3. Like this (replaced one line)?

    function Rabbit() {}
    Rabbit.prototype = {
      eats: true
    };
    
    let rabbit = new Rabbit();
    
    delete rabbit.eats;
    
    alert( rabbit.eats ); // ?
  4. The last variant:

    function Rabbit() {}
    Rabbit.prototype = {
      eats: true
    };
    
    let rabbit = new Rabbit();
    
    delete Rabbit.prototype.eats;
    
    alert( rabbit.eats ); // ?

Answers:

  1. true.

    The assignment to Rabbit.prototype sets up [[Prototype]] for new objects, but it does not affect the existing ones.

  2. false.

    Objects are assigned by reference. The object from Rabbit.prototype is not duplicated, it’s still a single object is referenced both by Rabbit.prototype and by the [[Prototype]] of rabbit.

    So when we change its content through one reference, it is visible through the other one.

  3. true.

    All delete operations are applied directly to the object. Here delete rabbit.eats tries to remove eats property from rabbit, but it doesn’t have it. So the operation won’t have any effect.

  4. undefined.

    The property eats is deleted from the prototype, it doesn’t exist any more.