Prototype Chain in JavaScript

In JavaScript, objects are linked to other objects through a property known as the prototype. This relationship forms a chain-like structure called the prototype chain, which is used when resolving property references.

When you try to access a property on an object, JavaScript will first check if the property exists on the actual object itself. If the property does not exist directly on the object, the JavaScript engine looks up the prototype chain, i.e., it checks the object’s prototype, the prototype’s prototype, and so on, until it either finds where the property is defined or reaches the end of the chain (which is typically the standard built-in Object prototype).

If the engine reaches the end of the chain and still hasn’t found the property, it returns undefined.

For instance, consider the following example:

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let animal = {
  eats: true
};

let rabbit = Object.create(animal);

console.log(rabbit.eats); // Output: true

Here, rabbit is an object created with animal as its prototype. When we try to access the eats property on rabbit, JavaScript checks if that property exists on rabbit. It doesn’t, so JavaScript looks up the prototype chain and finds the eats property on rabbit’s prototype, which is animal.

The prototype chain is integral to JavaScript and its importance can’t be overstated. It is the mechanism that enables JavaScript’s powerful and flexible object-oriented programming model, including its implementation of inheritance.

A cautionary note though: while JavaScript allows you to establish an arbitrary prototype chain, overly long or complex prototype chains can lead to performance issues and code that’s harder to understand and debug, so it’s generally considered best practice to keep them reasonably short and simple.

Comparison with Ruby Object Model

While both JavaScript’s prototype chain and the Ruby Object Model are used to achieve inheritance and property lookups, they operate on fundamentally different principles.

JavaScript’s prototype chain is based on prototype-based inheritance. In JavaScript, every object has a prototype, and objects inherit properties and methods from their prototype. The prototype chain is used when looking up properties: if an object doesn’t have a property, JavaScript checks its prototype for that property, and so on up the chain.

For example:

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let mammal = {
    vertebrate: true
};

let cat = Object.create(mammal);
console.log(cat.vertebrate);  // Outputs: true

The Ruby Object Model, on the other hand, is based on class-based inheritance. Ruby is a class-based object-oriented language, meaning that objects are instances of classes, and these classes define the behavior of their instances. When a method is called on an object, Ruby looks in the object’s class to find the method. If it’s not there, Ruby looks in the class’s superclass, and so on up the inheritance chain.

For example:

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class Mammal
  def vertebrate?
    true
  end
end

class Cat < Mammal
end

cat = Cat.new
puts cat.vertebrate?  # Outputs: true

In this Ruby example, the Cat class inherits from Mammal. When we call the vertebrate? method on a Cat object, Ruby first looks for that method in the Cat class. Not finding it there, it moves up the inheritance chain to the Mammal class and finds the vertebrate? method there.

So while they both provide a form of inheritance, JavaScript’s prototype chain and Ruby’s Object Model are based on different models of object orientation: prototype-based versus class-based.