What is the difference between Array.prototype.reverse and Array.reverse in Javascript?

Question

The question I have deals with an application of adding a new method to the existing String constructor. In Object Oriented Program for Javascript by Stoyan Stefanov, there is an example of using the Array constructor's .reverse() method to create one for the String constructor. Here is the example:

String.prototype.reverse = function() {
     return Array.prototype.reverse.apply(this.split('')).join('');
}

I thought the .reverse() method of Array belonged directly to the object of Array. In fact, when I try to do the second bit of code with this statement:,

String.prototype.reverse = function() {
     return Array.reverse.apply(this.split('')).join(''); //WITHOUT the .prototype
}

var rev = "karunesh".reverse(); //applying the code here

I get an error in the Firebug Console stating: "TypeError: missing argument 0 when calling function Array.reverse". That does not make any sense to me.

And of course, if I add back in the .prototype, it works perfectly fine.

Also, if is the case that I have to call upon prototype to access the .reverse() method from the Array object, then is it the case that I have to do that for any built-in object in Javascript?

Thanks for the help in advance!

Answer 1

Is it the case that I have to call upon prototype to access the .reverse() method from the Array object

No. To access a method on an object, just access it with dot notation. What you want to do is simply

return this.split('').reverse().join('');

That is just what apply (or call) does:

var arr = this.split('');
return arr.reverse.apply(arr).join('');

and finally arr.reverse === Array.prototype.reverse since that's where Array objects do inherit from. You are not accessing the reverse method on the Array constructor function object itself, you are to access the property that all Array instances share - via their prototype. Yet you hardly will ever need to use the prototype object explicitly, that's only when you're dealing with objects that are not Array instances (do not share the prototype) like arguments objects or NodeLists.

TypeError: missing argument 0 when calling function Array.reverse. That does not make any sense to me.

Array.reverse is a non-standard Array generic method which is only available in Firefox. It's purpose is to simplify the construct of applying Array prototype methods on other objects, and it does take the array-like object as it's first parameter. An example:

Array.reverse([0, 1]) // [1, 0]

which is equivalent to

Array.prototype.reverse.apply([0, 1]);

However, you were doing

Array.reverse.apply([…]/*, undefined*/)

which is calling the Array.reverse function with the array for the (irrelevant) this value and no actual argument, equivalent to

Array.prototype.reverse.apply(undefined)

and that throws the rightful exception.