JavaScript Hoisting: Variable declarations using var are treated as if they are at the top of the function (or global scope, if declared outside of a function) regardless of where these were placed.

example1.js

//Note the differences with both foo() functions

(function() {
    console.log(typeof foo);    // prints "function"

    foo();                      // logs "hello sencha"

    function foo() {
        console.log("hello sencha!");
    } 
})();

example2.js

//Note the differences with both foo() functions

(function() {
    console.log(typeof foo);    // prints "undefined"

    foo();                      // Error: foo is not a function

    var foo = function() {
        console.log("hello sencha!");
    } 
})();

//this difference is because the browsers interprets is at the code below:

(function() {
    var foo;
    console.log(typeof foo);    // prints "undefined"
    function __anon() { 
      console.log("hello sencha!"); 
    } //some anonymous function
    
    foo();                     // Error: foo is not a function

    foo = __anon;             //and now we bind anonymous function to foo 
})();

let-declarations.js

//ES2015
//Notice what happends when variables are defined with the let identifier:

(function() {
    console.log(typeof foo);      // ReferenceError: can't access lexical declaration `foo' before initialization
    
    foo();                         // I won't even get here.
    
    let foo = function() {
        console.log("hello sencha!");
    } 
})();