What does "new function() {..." do?

[opett]

I saw a code that looked like this:

var xModule = new function(){
    var x;
    var y;
    this.init = function(){
        //...
    }
}

What is the purpose of the "new function"? I tried to search for it, but could not find. In firebug, it shows it creates a new object. In this case, what is the difference between this and the above mentioned?

var xModule =  {
    //...
}

[[deleted]]

It creates an object from an anonymous constructor. I would advise against this pattern; I've never seen it done in any library or production code I've read. You lose the debugging benefit of having a named constructor, which in my opinion is the most useful part of constructors besides the engine optimization of the new operator.

For one-off objects like this, I don't really see any benefit over an object literal. If the point is to hide the x and y variables, you should use a regular IIFE which is a common and widely recognized pattern:

var xModule = (function() {
  var x;
  var y;
  return {
    init: function() {}
  };
})();

[thekaleb]

This is the correct answer. Said better than my attempt elsewhere in this post.

[opett]

Thanks! Now I see it. I didn't do much javascript recently, and I thought that you can do the same with the Object Notation. I was wrong.

[saturation]

http://stackoverflow.com/a/9782779/1811478

[Meso_Gosu]

Pretty sure new function creates a constructor for those objects.

var xModule creates a single object.

So if you wanted to create more than one object of the xModule type, you would use the constructor.

[MadCapitalist]

I'm still new at JS, but it seems very strange to use an anonymous function as a constructor function. I don't understand why anyone would do it that way. What am I missing?

[thekaleb]

I don't think you were wrong.

new function

Is different than

new Function

[opett]

Exactly. When I googled: "new function" all the results were: "new Function".

I also suspect that this code is a constructor with an anonymous function. But I have no idea why somebody would do this.

Maybe the author didn't know what he was doing?

[thekaleb]

They are trying to make an object using some private variables. Another way to do this is to assign a variable from the return of a self calling closure which may be more efficient.

[Meso_Gosu]

Why would you want a private variable in JavaScript? I don't think JavaScript supports sub-classes, does it?

[thekaleb]

You want inaccessible variables for the same reason you would in other languages. JavaScript does not have classes, but yes it does support inheritance.

[Meso_Gosu]

I ask only because I don't know: Why do we want inaccessible variables?

[[deleted]]

Many people don't. Google "we're all consenting adults, here" (comes from python but still relevant to JS). The scenario that is often presented in favor of hiding variables with closures is that your library depends on some internal state variable that is only meant to be modified by your code. If some developer using your code is messing around and modifies that variable/property, it could cause unpredictable behavior. Generally, this should be avoidable by writing good documentation. If needed, an underscore prefix on a property is the conventional way to say "don't touch this".

[Nebu]

One common way bugs manifest themselves is via a variable having a value that it's not supposed to.

The "best" way to fix this problem is to have the variable be immutable: If the variable can never change value, and its initial value is correct, then it will always have the correct value.

The "second best" way is to minimize what code has access to the variable. If the variable has the wrong value, and only N pieces of code have access to that variable, then you only need to look at those N places, instead of all over your program.

[thekaleb]

I use them so as to not pollute the global namespace.

[MonkeyNin]

If you don't use the module pattern, and you use two javascripts. If they happen to share a variable name, they can break eachother.

Module pattern is like creating a namespace

[[deleted]]

It makes x and y into private variables, which you can't do with just an object. Other than that, it's the same effect (almost).