When do I want to use "class" (static) methods or properties in JavaScript?

Question

In JavaScript, why do you need to bind properties directly to the constructor?

var Human = function() {}; Human.specie = "Homo Sapience"; 

I have this question after looking at the CoffeeScript __extend helper function that contains among the lines:

 for ( var key in parent ) { if ( __hasProp.call( parent, key ) ) child[key] = parent[key]; } 

which copies the properties / methods for the subclass object directly from the constructor object. But why would anyone do this?

Thanks!

Answer 1

In short, the answer to the question posed is the distance between the names . There are certain values ​​that may make sense for sharing in your program and are semantically related to a particular class. These functions and values ​​can simply be placed in some variables, but their binding to a design function is practical in order to put them down.

The best example is the JavaScript Math class (for purists, I know this is not really a class, this is an object):

 // There are constants Math.E Math.PI Math.SQRT2 // And there are also methods Math.ceil Math.cos Math.sin 

Thus, the methods and values ​​(stored in constants) are always the same, and they do not depend on the instance on which they are called, and it makes no sense to have them on instances.

Answer 2

( Edit: In the original form, the question is about attaching properties to classes and attaching them to prototypes, so what I answer.)

This is indeed more arbitrary than anything else. If you write

 Human::specie = "Homo sapiens" 

(where Human::specie is CoffeeScript short for Human.prototype.specie ), then declare jane = new Human , then jane.specie will be "Homo sapiens" (unless you specifically set jane.specie to something else). In this case, it sounds desirable.

But in other cases, having a property shared by a large number of prototypes makes it difficult to understand your code. Say you have a Logger class with a config object. If you attach this object to a prototype, you can write code like this:

 log = new Logger log.config.destination = './foo' 

This will change the assignment of all Logger instances to './foo' because there is only one config object. If you want config applied to all instances of Logger , you must attach it to the class, removing the ambiguity from the above code:

 log = new Logger Logger.config.destination = './foo' 

Answer 3

The game says that you have an object called the world. However, the game will only have one world. This is theoretically the reason why you do this.