js-inflector v0.0.1

var inflect = inflector.start();

var capital = inflect(someValue, 'capital');  
//--> Tree

Modify multiple word states at once:

var pluralAndCapital = inflect(someValue, ['capital', 'plural']);  
//--> Trees

Set a group state:

inflector.group({
  'properPlural': ['capital', 'plural']
});

With a group state:

var pluralAndCapital = inflect(someValue, 'properPlural');  
//--> Trees