God saw their works, that they turned from their evil way. God repented of the evil which he said he would do to them, and he didn't do it.
Notice that in this section of verses (Jonah 3:5-10) the name "God" (Heb. Elohim, the strong one) appears exclusively. However the name "LORD" (Heb. Yahweh, the covenant keeping God) occurs frequently earlier and later in the story. Jonah did not present God, and the Ninevites did not fear God, as the covenant keeping God of Israel but as the universal Supreme Being. Likewise God did not deal with the Ninevites as He dealt with His covenant people Israel but as He deals with all people generally. Thus the story teaches that God will be merciful to anyone, His elect and His non-elect, who live submissively to natural divine law (cf. Genesis 9:5-6).