Global and Local and Private Functions (Javascript)

1. None, People use "private" because they are mistaken and are meant to say "local"

2. local variables are defined as

var foo = "local";

global variables are a properties of the global scope object (which is window in the browser)

window.foo = "global";

The fact you can do foo = "global"; without first declaring variable foo with var foo is a "bug". This is fixed in ES5 strict mode.

(function () { "use strict"; foo = 42; })()

gives ReferenceError: foo is not defined

Note that you can make variables global by declaring them in outer most scope

var foo = "global";
function bar() {
  var foo = "local";
}

It should be noted that you shouldn't have any code in outer most scope, ever. You should be wrapping all your scope in anonymous functions so that you get "module level scope". This means you have a per file based top level scope. This is part of the module pattern.

In the context of the browser, the var keyword scopes the variable to that of the current function.

var a = 10;

var b = function(a) {
   console.log(a);  # 15
}

b(15);
console.log(a);  # 10

If you do not include the var keyword, it is assigned the scope of window and is considered global. Unless you have a very good reason to exclude it, always include the var keyword.

A variable is considered private if it only exists inside a function scope. This commonly takes the form of an anonymous function. This is not actually a private variable in the common sense of the term, it is simply a local variable.

(function() {
  var x = 10;
})();

console.log(x); #undefined