The ECMAScript standard (edition 2) defined the finally keyword and reserves it for future use. At edition 3, it was made a required keyword.
Refer to the try ... catch ... finally topic for more details.
ECMA 262 edition 2 - section - 7.4.3
ECMA 262 edition 3 - section - 7.5.2
ECMA 262 edition 3 - section - 12.14