javascript string is Immutable

>>> s = 'hello world'
"hello world"
>>> s[0] = 'a'
"a"
>>> s
"hello world"

http://stackoverflow.com/questions/51185/are-javascript-strings-immutable-do-i-need-a-string-builder-in-js

from the rhino book:

In JavaScript, strings are immutable objects, which means that the characters within them may not be changed and that any operations on strings actually create new strings. Strings are assigned by reference, not by value. In general, when an object is assigned by reference, a change made to the object through one reference will be visible through all other references to the object. Because strings cannot be changed, however, you can have multiple references to a string object and not worry that the string value will change without your knowing it[1]