Charles Spurgeon Commentary 1 Samuel 18:4-5

"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. And David went out whithersoever Saul sent him, [and] behaved himself wisely: and Saul set him over the men of war, and it was good in the sight of all the people, and also in the sight of Saul`s servants." — 1 Samuel 18:4-5 (ASV)

And gave it to David, and his garments, even to his sword, and to his bow, and to his girdle. And David went out whithersoever Saul sent him, and behaved himself wisely: and Saul set him over the men of war,

Probably, Jonathan had previously occupied that position; but now that David is called to supplant him, Jonathan is not jealous of him, but he loves him as he loves his own soul.