Prototyping with a function assignment in Javascript

Question

I have been trying some fundamentals in javascript and here is what I observed.

I wrote a prototype modification method for Function object

Function.prototype.method = function(name, func)
{
   this.prototype[name] = func;
   return this;
}

function pirates(value)
{
   console.log("I just throw away the value!!" + value);
}

pirates.method("my_skill", function(){
   console.log("Pirate skills");
});

new_pirate = new pirates(1234);

//SUCCESS
new_pirate.my_skill(); //prints "Pirate skills"

var someCrappyVariable = function(){
  return function()
  {
     console.log("I am going to just sit and do nothing. Really!");
  }
}();

**//Throws error. WHY???????? This was assigned a function, so ideally prototype should work on this too? Shouldn't it?**
someCrappyVariable.method("crappyFunction", function(){
   console.log("am I doomed?");
});

Why does last assignment throws an error that someCrappyVariable is not a function, when it was assigned a function reference earlier? I am confused.

Cheers

The error appears for me when trying to execute the crappyFunction method — Justin Johnson
– Justin Johnson, Commented Dec 28, 2009 at 21:04

James · Accepted Answer · 2009-12-28 19:06:33Z

2

Your code should be throwing an error here:

new_pirate.method("my_skill", function(){
   console.log("Pirate skills");
});
// TypeError: new_pirate.method is not a function

... since method is not a direct or inherited method. The object created from new pirates(1234) will not inherit from Function; it will inherit from Object (which, ideally, shouldn't be extended with custom methods).

answered Dec 28, 2009 at 19:06

James

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3 Comments

artificialidiot Over a year ago

It doesn't work as described. It really throws an error on "my_skill".

Priyank Over a year ago

That bit is right. So do I assume that problem with my someCrappyVariable also not being able to inherit the prototype method is that, that it's inheriting from object and not from function.. Why does it inherit from object? Did we not assign a function to it?

artificialidiot Over a year ago

new invokes a function as a constructor method over a newly created object. There is one more error in your code which I'm gonna post as an answer.

artificialidiot · Accepted Answer · 2009-12-28 20:09:16Z

-1

If we remove the code for "new_pirate", there is still an error.

In standard js, a prototype chain for an object can be established by assigning the prototype property of the constructor before it runs and creates the object. So if you call the "method" on someCrappyVariable, it will set the "name" on the "prototype" property of the created object only, which is pretty much useless.

What you actaully want is to set someCrappyVariable.constructor.prototype[name] so all object with the same constructor can find the set method like someCrappyVariable[name] (which is created by an anonymous function in this case).

answered Dec 28, 2009 at 20:09

artificialidiot

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