Charles Ellicott Commentary 1 Samuel 18:4

"And Jonathan stripped himself of the robe that was upon him, and gave it to David, and his apparel, even to his sword, and to his bow, and to his girdle." — 1 Samuel 18:4 (ASV)

Gave it to David. —It has been suggested that the reason for this gift was to enable his friend David—then poorly dressed—to appear at his father’s court in a fitting dress; but this kind of present was usual among friends in those distant ages. Glaucus and Diomed, for instance, exchanged armor of a very different value.

“Let us now exchange arms, and prove to each host
We guard the friendship of the lineage we boast.

For Diomed’s brass arms, of humble make,
For which nine oxen were paid (a common price),
He gave his own of gold, divinely crafted:
A hundred oxen the shining purchase cost.”

Iliad, vi. 286–295.