Javascript Scope Of Function Declarations

The var keyword in javascript causes a variable to be stored in the local scope. Without var variables belong to the global scope. What about functions? It's clear what happens

Solution 1:

It belongs to the current scope, always. For example:

// global scope

// foo is a global function
function foo() {

    // bar is local to foo
    function bar() {

    }

}

Regarding your second question, this:

foo = function() {...}

is an anonymous function expression assigned to a global variable (unless you're running is strict mode, then foo would be undefined). The difference between that and function foo() {} is that the latter is a function declaration (versus a variable declaration, which is assigned an anonymous function expression).

You might be interested in this excellent article about function declarations and function expressions: Named function expressions demystified.

Solution 2:

Function declarations are always local to the current scope, like a variable declared with the var keyword.

However, the difference is that if they are declared (instead of assigned to a variable) their definition is hoisted, so they will be usable everywhere in the scope even if the declaration comes in the end of the code. See also var functionName = function() {} vs function functionName() {}.

Solution 3:

Noteworthy distinction taking implicit globals into account:

var foo = function() {
  // Variables
  var myVar1 = 42;  // Local variable
      myVar2 = 69;  // Implicit global (no 'var')

  // Functional Expressions
  var myFn1 = function() { ... }  // Local 
      myFn2 = function() { ... }  // Implicit global

  function sayHi() {
    // I am a function declaration. Always local.
  }
}

Hopefully that clarifies a little. Implicit globals are defined if you forget a var before your assignment. Its a dangerous hazard that applies to variable declarations and functional expressions.

Solution 4:

Your first example (var foo = function() {...}) is called an anonymous function. It is dynamically declared at runtime, and doesn't follow the same rules as a normal function, but follows the rules of variables.