John Gill Commentary 1 Samuel 20:4

"Then said Jonathan unto David, Whatsoever thy soul desireth, I will even do it for thee." — 1 Samuel 20:4 (ASV)

Then said Jonathan unto David
Now giving credit to what he had said, and in order to comfort and support him under the apprehensions he had of danger:

whatsoever your soul desireth, I will even do [it] for thee ;
for the preservation of his life, by speaking to his father on his behalf, endeavouring to dissuade him from his evil intentions, or by hiding and concealing him in some obscure place, that he might not execute his evil designs upon him, or by any method he could point out to him.