Albert Barnes Commentary 1 Samuel 17:45

"Then said David to the Philistine, Thou comest to me with a sword, and with a spear, and with a javelin: but I come to thee in the name of Jehovah of hosts, the God of the armies of Israel, whom thou hast defied." — 1 Samuel 17:45 (ASV)

A shield - “A javelin,” see 1 Samuel 17:6 note.