Privacy in JavaScript

Question

Function scoping offers the only privacy in JavaScript.

So the canonical:

function Ctor(dep1, dep2) {
  this._dep1 = dep1;
  this._dep2 = dep2;
}

Ctor.prototype.foo = function() {
  // use this._dep1/2...
}

...is problematic in that it offers no encapsulation for the injected dependencies.

An alternative (albeit slightly different in terms of location of foo) that offers real encapsulation might be:

function factory(dep1, dep2) {
  return {
    foo: partial(foo, dep1, dep2), // or use bind (partial could be a library fn for partial application)
  };
}

function foo(dep1, dep2) {
  // use dep1/2
}

But I rarely see this pattern. Is there a good reason to not use the latter?

Answer 1

Simple: why?

"Privacy" is terrifically overrated. It doesn't hide anything from anyone who really wants it. Having "protected" or "private" members is purely for the programmer's benefit, specifically to indicate how a particular member is supposed to be used (this here is a public API, this here is not so please don't touch it unless you know what it does). That's all. A naming convention like starting underscores is perfectly enough to implement that. If a member name starts with an underscore, don't touch it, unless you know what you're doing.

There's no intrinsic need to bend over backwards further than that.

Answer 2

You already state the advantages of the second pattern. The disadvantages are:

• you must either group methods by visibility (even if the private method is intimately related with a public one, but barely related to other private methods), or repeat all public methods in the object literal (which doesn't work well with jsdoc).

• the code introduces a separate function object for every instance of your class, which sacrifices some performance (this usually doesn't matter, but sometimes might)

• unlike private modifiers in many programming languages, this encapsulation can not be circumvented, even if misapplied and I really know what I am doing (historically, JavaScript has been an environment where anything goes, and many attribute its success to this extensibility)