Porus didn't know it at the time but he was going to face a rather tired and dispirited Macedonian army. Alexander's army had been wandering for years before finally getting custody of Spitamenese's defeat, his head, and ensuring peace accords. This campaign had taken two years and severely stunted chances of a proper India Invasion. His army had taken a noticeable toll during the endless back/double back again type campaign led by guerrilla leader/self titled King of Asia called Spitamenese.
After crossing the Hindu Kush I believe he fought another pretty big battle at the Siege of Aronos. Porus had an idea he was coming so he had been trying to build a coalition to properly fight a potentially significant battle in defense of his surrounding kingdoms. Porus had a great natural defense working for him. Porus was located east across a large river system from Alexander's army. This was perhaps the Porus's best asset and I'm sure he was anxious to see it actually work. Alexander would have ford a wide, powerful river during imminent rains. Make no mistake, this was a complex amphibious style assault accompanied with artillery weapons and siege engines. Once the Macedonian troops landed on the eastern bank they would be on death's ground and fighting for victory or total defeat.
After a while of deceit and deception, Alexander crossed north of Porus's main camp. The main Macedonian body, I believe, would attack the main force of Porus's. Getting his army across the river proved difficult because they misread the proper layout of river and landed instead on an island. They would have to ford on more time and sumerly land in hostile territory. The battle happened was initially quick and hard-fought, as well as confusing for both sides. But, the Porus detachment that met Alexander's force initially did some damage but the ensuring cost the lives of some top Indian leaders. But, the Macedonian main force was caught off guard by Indian heavy archers and battle elephants. Confusion and screams were coming from everywhere and finding a gap for both armies to exploit for attack was difficult. Many were falling on both sides but Macedonian axes and sarissas proved to be a good deterrent to the battle elephants. Porus's main body started to collapse and one more charge led by the calvary ended the battle with the Macedonians gaining a (premature expression) Pyrrhic victory, of sorts.
The battle of Hydaspes was a victory for Alexander but the India invasion was over. The Macedonians mutinied shortly after the battle after Alexander quickly ordered a bigger invasion deep into interior India. But his army had already performed beyond the limits any other army in history and would go no further. This must have left a bad taste in his mouth as he thought the Eastern Sea could be reached in a relatively short time. So, Alexander instead built a good sized navy too navigated his troops, support by two detachments flanking the opposite river banks, down too the eventual coast. This was more or less tactical retreat down the Indus River systems. But, make no mistake, this was also a military operation to secure his eastern border from Jhelum area all the way down to the coast and conquer any tribal holdouts.
This was not an easy campaign and they experienced some fierce battles. Alexander experienced his worst combat wound in his career during this campaign which may have caused a lingering effect on him. If Porus had defeated Alexander's army he would not have had a safe refuge to have built a capable navy and had ordered a systematic retreat. If Porus did win the battle and defeated Alexander's army, I doubt he would have provided him with access to boats and an uninterrupted launch date. Porus had his kingdom returned to his rule with the price of tribute. Him effectively killing the moral and willing participation of the Macedonian army to conduct further military engagements doomed what could have been Alexander's greatest discovery and crowning achievement. With Alexander now going home and no threat of further hostile combat against the Indians, this would provide a new generation of Indian Kings to rise up and take back what Alexander had gained before his retreat.