Extending vs. Overwriting JavaScript Prototypes: Which Approach is Best?

Defining a JavaScript Prototype: Understanding the Differences

Defining object prototypes in JavaScript is essential for inheritance and code reusability. However, there are two common approaches to prototype definition: extending the existing prototype or overwriting it entirely.

Option 1: Extending the Existing Prototype

Person.prototype.sayName = function(name) {
   alert(name);
}

This approach extends the current prototype object Person.prototype by adding a new method sayName. Existing instances of Person will inherit this new method.

Option 2: Overwriting the Prototype

Person.prototype = {
   sayName: function(name) {
      alert(name);
   }
}

Unlike Option 1, this approach assigns a new prototype object to Person.prototype, overwriting the original one. Only future instances of Person will inherit the sayName method.

Functional Differences and Benefits

The primary functional difference is that Option 1 allows existing instances to access the new method, while Option 2 only affects future instances.

Another difference is that by overwriting the prototype (Option 2), you inadvertently discard any other properties or methods previously defined in the original prototype. Although these typically include only the constructor property, it's worth noting this potential downside.

Recommendation

Option 1 (extending) is generally considered more concise and robust as it ensures that all instances of Person can access sayName. Avoid Option 2 unless you need to reinitialize the prototype for specific reasons.

Alternative Approach

If the object literal syntax of Option 2 appeals to you, consider using Object.assign to extend the existing prototype:

Object.assign(Person.prototype, {
   sayName: function(name) {
      alert(name);
   }
});

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