Javascript function scoping and hoisting

Function hoisting means that functions are moved to the top of their scope. That is,

function b() {     a = 10;     return;     function a() {} } 

will be rewritten by the interpeter to this

function b() {  function a() {}  a = 10;  return;}

Weird, eh?

Also, in this instance,

function a() {}

behaved the same as

var a = function () {};

So, in essence, this is what the code is doing:

var a = 1;                 //defines "a" in global scopefunction b() {     var a = function () {}; //defines "a" in local scope    a = 10;                 //overwrites local variable "a"   return;      }       b();       alert(a);                 //alerts global variable "a"

What you have to remember is that it parses the whole function and resolves all the variables declarations before executing it. So....

function a() {} 

really becomes

var a = function () {}

var a forces it into a local scope, and variable scope is through the entire function, so the global a variable is still 1 because you have declared a into a local scope by making it a function.

The function a is hoisted inside function b:

var a = 1; function b() {    function a() {}    a = 10;    return;} b(); alert(a);

which is almost like using var:

var a = 1; function b() {    var a = function () {};   a = 10;    return;} b(); alert(a);

The function is declared locally, and setting a only happens in the local scope, not the global var.