Albert Barnes Commentary Jonah 4:2

"And he prayed unto Jehovah, and said, I pray thee, O Jehovah, was not this my saying, when I was yet in my country? Therefore I hasted to flee unto Tarshish; for I knew that thou art a gracious God, and merciful, slow to anger, and abundant in lovingkindness, and repentest thee of the evil." — Jonah 4:2 (ASV)

And he prayed to the Lord - Jonah, at least, did not murmur or complain of God. He complained to God about himself. He expostulates with Him. Shortsighted indeed and too wedded to his own will! Yet his will was the well-being of the people whose prophet God had made him. He tells God that this was what he had dreaded all along. He softens it, as well as he can, by his words, “I pray You,” which express a prayer against this outcome and a spirit of unsubmissiveness. Still, he does not hesitate to tell God that this was the cause of his first rebellion! Perilous to the soul it is to speak of former sin without penitence; yet it is to God that he speaks, and so God, in His wonderful condescension, makes him teach himself.

I knew that Thou art a gracious God - He repeats to God, to the letter, His own words from Joel 2:13. God had thus revealed Himself anew to Judah. He had, doubtless, on some repentance which Judah had shown, turned away the evil from them. And now by sending him as a preacher of repentance, He implied that He would do the same to the enemies of his country. God confirms this by the whole sequel.

Therefore, from that time on, Israel knew that God was also intensely and infinitely full of gracious and yearning love toward the pagan; indeed (as the grammatical form rather implies), He was mastered (so to speak) by the might and intensity of His gracious love, being slow to anger and delaying it, great in loving tenderness, and abounding in it. They also knew that toward them, when evil was about to be inflicted, or had been partially or wholly inflicted, He would repent of that evil and replace it with good on the first turning of the soul or the nation to God.