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Comment by tkiley | Hacker News Reader

Excellent write-up! I've learned most of these things the hard way :/ I'm filing this away to recommend to any developers who are setting out to use Javascript extensively for the first time.

One quibble: In the "common pitfalls" section regarding the "this" object, they say that locally-defined functions within other functions never have any practical use. I might disagree: with a little coaxing, you can convince locally variables inside the constructor (both functions and other variables) to serve as private properties of an object; this is the only technique I know that allows for private properties.

(I haven't actually done this in code that has been maintained/used anywhere, I just did it as an experiment and filed it away as a "that's cool" for future reference)

Edit: Here is an example of what I'm talking about: https://gist.github.com/866103

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> One quibble: In the "common pitfalls" section regarding the "this" object, they say that locally-defined functions within other functions never have any practical use.

Might also come in handy if you are into functional programming with Javascript.

There is actually a much better way to make private functions and properties in JavaScript using the module pattern and closure.

Since all functions are objects in JS, you can create a function with some internal (private) functions and properties than return a object with some publicly accessible functions. A basic example:

https://gist.github.com/866343

Closure gives you a really powerful structure that lets you encapsulate your code and then you don't have to use this and risk clobbering the global window object :)

It's basically the same thing, closure is the name of the game here. Your solution is probably a bit more costly for memory though.

Be careful of that. There is a trap right there. Your example is using the "new" keyword and not executing the function. So what happened?

1. If you use the "new" keyword, and don't execute the function. "x = new foo()". X becomes an "object". "foo()" is behaving like a class. You got to define the properties and methods of this class with the "this" keyword. Once you create your object with the "new" keyword, these variables got assigned to the object. And better, you can access them with the prototype.

2. If you execute the function in your code, that is you put "foo()": Open your FireFox with FireBug and notice two new global variables in the Windows object "get_my_private" and "set_my_private".

So it depends on the usage. "this" insides of a function is useful, if your intent is to use the function as a class. If not, it's dangerous, as the variables becomes global and may interfere with other variables.

There's an easy way to fix that, though:

  function Foo(){
    if (this == window) throw "USE NEW!";
    // continue with object creation
  }

Or use the standard practice of having class-creating function names capitalized, and let people know to follow it.

Fix: ( from http://elegantcode.com/2010/12/21/basic-javascript-part-4-en... )

    function Podcast() {
      if(false === (this instanceof Podcast)) {
        return new Podcast();
      }
      // other code
    }

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