Version 5 of ECMAScript introduced a widely-used factory pattern for creating objects and managing inheritance, known as Object.create()

In ECMAScript 5, the factory pattern for construction and inheritance is defined as Object.create(). By passing in an object to inherit from, a new object is returned with all connections properly set up.

Let's look at this code snippet:

var x = {
    text: "Bonjour",
    speak: function(){ alert(this.text); }
};
var y = Object.create(x);

Is my understanding correctly summarized below?

When y is created, it inherits a property called text from its prototype. This means that the property doesn't exist directly on the instance (y.text), but is actually stored on the prototype (y.prototype.text). If we try to access the value of y.text at this point, JavaScript will search for it implicitly, find it on the prototype, and return it.

y.speak(); // alerts Bonjour

However, if I assign a new value to y.text, it does not affect the inherited y.prototype.text. Instead, it creates a new instance property y.text which takes precedence over the inherited y.prototype.text when accessed in the future. Why is this so?

y.text = "Monde";
x.speak(); // still alerts Bonjour              >> Why hasn't this changed?
y.speak(); // now alerts Monde

Answer №1

The reason for this occurrence is due to the concept of the prototype chain. When JavaScript runtime searches for a property within an object, it first checks its own properties. If the property is not found, it then continues searching on its prototype, and so forth.

It's important to note that you aren't actually overriding a property, but rather adding a new one to the entire object which conceals the property from the prototype. Once again, this behavior can be attributed to how the prototype chain functions.

Answer №2

Understanding how javascript property lookup and assignment work is key. Updating a prototype property involves creating an object inside the prototype object.

var proto = {
    fields: {
        phrase: 'Hello'
    },
    say: function () { console.log(this.fields.phrase) }
};

var a = Object.create(proto);

a.fields.phrase = 'World';
proto.say();
a.say();

But what exactly is happening here?

When a.fields.phrase = 'World' is executed, it is essentially equivalent to:

var tmp = a.fields;
tmp.phrase = 'World';

a.fields === a.__proto__.fields // true

This explains why the property was successfully updated in the prototype.

In your example, assigning a value directly to an object simply instructs the JavaScript engine to assign the value "World" with the key "phrase" to the object a. It's all part of how objects work in javascript.

For more detailed information on how objects operate in javascript, click here.

Answer №3

Let's start by examining this particular statement

var b = Object.create(a);

This code snippet is responsible for creating a brand new object, which we can refer to as newObj. Here is what takes place after this:

  • b now points to the newly created object newObj.

  • newObj is connected to the properties and methods of object a through its [[prototype]] reference.

At this point, newObj does not possess any unique methods or properties.

Consider b.Say() --
When you call this method, b will first look towards newObj, checking if it contains the function Say(). Since it doesn't exist in newObj, the lookup moves up the [[prototype]] chain. As newObj's [[prototype]] link leads to object a, the method is found in a's object. It gets executed with newObj as its context, resulting in the output of 'Hello'.

Now let's discuss b.Phrase='world'
By setting the property Phrase on newObj (pointed to by 'b'), any future calls to b.Phrase will immediately find the value within newObj itself, without needing to traverse the [[Prototype]] chain (i.e., referencing a's object).

Lastly, examining Final b.Say()
Given that newObj lacks the Say() method, the search follows the [[prototype]] chain upwards to locate the method and execute it within the scope of newObj. With newObj possessing the phrase property, this.phrase returns 'world'

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