forthwith vs straightaway

forthwith is not a noun while straightaway is a noun.

Word	Noun	Adjective	Verb	Adverb
forthwith	No	No	No	Yes
straightaway	Yes	Yes	No	Yes

As adverbs, forthwith and straightaway are synonyms defined as:

forthwith and straightaway: without delay or hesitation; with no time intervening

Other synonyms of forthwith include at once, directly, immediately, instantly, like a shot, now, right away, straight off.

forthwith (noun)	straightaway (noun)
	a straight segment of a roadway or racecourse

forthwith (adjective)	straightaway (adjective)
	performed with little or no delay

forthwith (adverb)	straightaway (adverb)
without delay or hesitation; with no time intervening	without delay or hesitation; with no time intervening