Strings are immutable

gistfile1.js

var str = "foo";             // Literal String
str.replace('f', 'o');       // Modified the string     
str;                         // return "foo" not "ooo" 

// similar strings are equal 
var s = "abc";
var d = "abc";
d == d;                     // return true

// String methods like charAt, indexOf

var s = "foo";
s.charAt(0);                // return "f"
s.indexOf("f");             // return 0