bric-a-brac vs knickknack

bric-a-brac and knickknack both are nouns.

Word	Noun	Adjective	Verb	Adverb
bric-a-brac	Yes	No	No	No
knickknack	Yes	No	No	No

As nouns, bric-a-brac and knickknack are synonyms defined as:

Other synonyms of bric-a-brac include knickknackery, nicknack, whatnot.

bric-a-brac (noun)	knickknack (noun)
miscellaneous curios	miscellaneous curios
	a small inexpensive mass-produced article