Can you overcharge your iphone 4s?

I leave my iPhone 4s charging overnight so will the constant charge every night affect the battery life? Also, what is the average battery life on a 4s.

Like all batteries, charging them longer than they need can negatively affect the batteries overall lifespan. It's best to always allow your batteries to deplete before recharging.

The lithium ion battery don't get overcharged… The battery stop charging once it reach the 100%, it use like 3% of the charge and then start recharging again, this cycle keep the ions working and prevent them from over charging.

like all battery they get affected with time, i have my iphone for 2 years, charge it overnight since day 1 and the battery still work like when i got it out of the box.

No. Apple devices have a mechanism in place to prevent that. When it reaches its max charge it stops charging.