Placing JavaScript functions inside of an object of a variable -

i see quite often, i'm not sure purpose of or advantages (if any). placing function or sets of functions inside of object, value variable.

var variablename = { (function() {do stuff} )} 

javascript uses global variables. means every script cann access , modify variables , possible overwrite variables same name. secure own script, can use global variable (like var myscript=... ) , write it. give example, wrote simple script navigate between html-files. can see variable called 'navi' capsulates stuff inside. contains variables , functions , (in case) access allowed returned names. if wrote functions next , previous out of navi, may overwritten in other scripts.

var navi = (function(){     var currentpage = 0;     var pages = [];     pages[0] = "first.html";     pages[1] = "second.html";     pages[2] = "third.html";      var next = function next(){         if(currentpage < pages.length - 1){             currentpage++;             location.href = pages[currentpage];         }     };      var previous = function previous(){         if(currentpage > 0){             currentpage--;             location.href = pages[currentpage];         }     };      return{         next: next,         previous: previous,         currentpage: currentpage,         pages: pages     }; })(); 

to call html file:

<a href="javascript:navi.previous();">previous page</a> <a href="javascript:navi.next();">next page</a> 

so far.