1 Chronicles 24:2

“But Nadab and Abihu died before their father, etc.] While he was living, (see <031001>Leviticus 10:1,2) and had no children, therefore Eleazar and Ithamar executed the priests’ office: otherwise, as Jarchi observes, had their brethren left children, they would have come into the office before them.”

“Nadab and Abihu died before their father--that is, not in his presence, but during his lifetime (see Num 3:4; Num 26:61). therefore Eleazar and Ithamar executed the priest's office--In consequence of the death of his two oldest sons without issue, the descendants of Aaron were comprised in the families of Eleazar and Ithamar. Both of these sons discharged the priestly functions as assistants to their father. Eleazar succeeded him, and in his line the high priesthood continued until it was transferred to the family of Ithamar, in the person of Eli.”

“Therefore, &c. - Were the only persons to whom the execution of that office was committed. 1 Chronicles 24:3”

“Nadab: Exo 24:1, Exo 24:9 died: Lev 10:2; Num 3:4, Num 26:61 Eleazar: Exo 29:9; Lev 10:12; Num 16:39, Num 16:40, Num 18:7 1 Chronicles 24:3”

“Cf. Lev 10:1., Num 3:4. These priestly families David caused ( Ch1 24:3) to be divided, along with the two high priests (see on 13:16), "according to their service." פּקדּה, office, official class, as in Ch1 23:11. 1 Chronicles 24:4”