String toUpperCase()

Converts all characters in this string to upper case. If the string is already in all upper case, this method returns this.

'alphabet'.toUpperCase(); // 'ALPHABET'
'ABC'.toUpperCase();      // 'ABC'

This function uses the language independent Unicode mapping and thus only works in some languages.

Source 

// TODO(floitsch): document better. (See EcmaScript for description).
String toUpperCase();