CODEWITHSUNDEEP
javascriptconstructorprototype
Goodword
Goodword

Reputation: 1645

Why is this property defined on the prototype--not on the constructor?

In the chapter on Bugs and Error Handling in Eloquent Javascript, I found the following piece of code. I would like to know why the writer set the name property on the prototype--not in the constructor. 

function InputError(message) {
  this.message = message;
  this.stack = (new Error()).stack;
}
InputError.prototype = Object.create(Error.prototype);
InputError.prototype.name = "InputError";

Throughout the book, he has been defining properties in the constructor and methods on the prototype. Nicholas Zakas recommends doing the same in Ch6 of Professional Javascript for Web Developers, because, he says, if you define methods in the constructor, then each call to the constructor creates a new instance of the method. This is of course the opposite situation--a property being defined on the prototype.

To reiterate: What I want to know is, why might the author have defined 'name' property on the prototype?

So far, I've read these two SO posts two no avail.

What is the difference between assigning a function directly to a constructor versus to it's prototype, and why?

-This told me what I've already read in the books mentioned above.

Why defining properties in the prototype is considered an antipattern

-I didn't really understand this post, but I think it was irrelevant, because it was talking about patterns in other languages. I am concerned with javascript.

Upvotes: 0

Views: 59

Answers (2)

Goodword
Goodword

Reputation: 1645

Every InputError shares the same name. The only thing that really needs to change in each new instance is the message argument. In this case, it doesn't really matter.

Upvotes: 0

Felix Kling
Felix Kling

Reputation: 817208

Why might the author have defined 'name' property on the prototype?

Because every instance of InputError shares the same name. "Methods" is just a colloquial term for properties who have a function as value (*). Usually it is those methods that should be shared between instances, but in some cases (like this one) it might be data as well.

*: Meaning, there is no difference between between a property and a method.

Upvotes: 2

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