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Des concepts simples JavaScript H?ritage prototypal

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If you have written a bit of JavaScript, you certainly have heard the word prototype. Prototype is not only a JavaScript library, but also a key concept of JavaScript, which we will explore in depth today, without making your head spin.

(Almost) every object in JavaScript (where everything is an object, even functions) has an internal prototype property, which points in fact to a prototype object. When a method or property is missing on an object, it is searched on its prototype, and if not found on the prototype's prototype, and so on. the last object of the chain usually is Object.prototype.

Sometimes the prototype is exposed with the __proto__ property, although this is not part of the standard. According to the EcmaScript specification, it is a private property.

What you access with MyClass.prototype is instead a different thing. When you instance an object with new MyClass(), the object will its __proto__ set to MyClass.prototype; but MyClass.prototype is not in the prototype chain of MyClass. Its __proto__ instead is likely to be Function.prototype, since classes in JavaScript are defined as functions.

Only functions in JavaScript have this public property, which in fact can only be used for the valorization of the internal __proto__ of objects created by these functions.

A cool (but also dangerous) thing is that a reference is kept to MyClass.prototype, so you can add methods that work on *this* to MyClass.prototype later, and they will show up on every instance of MyClass, being shared by all of them.

Of course, since MyClass.prototype is an object just like everything else, its chain ultimately falls back to Object.prototype.

To implement classical inheritance, you have to leverage the prototype chains. Since properties and methods are searched on an object's __proto__, if you want it to inherit from another object you have to modify the prototype of its class (you can't modify __proto__).

Each instance of Dog now will inherit member fields from the new Animal() and consequently from Object.prototype.