While I've never used a language that had built-in variable privacy, a book I'm reading by Douglas Crockford explains a way to create privacy in JavaScript, however it doesn't make sense to me so. The term private isn't something I think of as even being possible in a programming environment, since everyone working on the code can potentially change any mutable object or method right? So the two things that don't make sense to me arehow privacy is helpfulhow privacy is defined - fundamentally one developer can't keep something in source code absolutely private (untouchable, as I understand it) from another developer, so where does one consider the variable private? When it's hard to reach? Can someone explain this, possibly with a real-world example? 
What is the usefulness of private variables?
encapsulation
Within software development, privacy (of software entities) is usually defined as restricting access to that variable/function/method.If a variable is private, then only functions or methods that belong to the same class or module are allowed/supposed to access that variable.As you see, privacy here is completely unrelated to which developer is writing the code, but rather is is about which parts of the code have access to which other parts of the code.The advantage of private variables or functions is that, by restricting which parts of the code have access, you can more easily reason about how the variable gets used and who will be affected if you change it.