Charles Ellicott Commentary 1 Samuel 17:50

"So David prevailed over the Philistine with a sling and with a stone, and smote the Philistine, and slew him; but there was no sword in the hand of David." — 1 Samuel 17:50 (ASV)

But there was no sword in the hand of David. —The story of the daring of the son of Jesse rightly emphasizes the extraordinary courage and skill of the young champion of Israel.

If his heart had failed him for one instant—as indeed, it very well might have; if he had not possessed an unshakeable confidence in an unseen Helper; or if his skill as a marksman had failed him in the slightest degree, the Philistine would have laid David lifeless at his feet with one blow.

Alternatively, if the active shepherd boy had eluded his giant antagonist, it would have had to be by flight. In any case, the single combat upon which Israel had staked so much would have gone against the chosen people.