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No, not necessarily, but it can cause a minor issue. iPhones use a Lithium-Ion battery for its power, using while it's charging won't necessarily ruin the battery, but if you're using the phone while the battery is in the charging process, it can cause the phone to think it has more power than it actually has.
The truth: Your phone is smarter than you think. Once it's fully juiced up, it knows to stop charging. That means the battery isn't even in use at all. However, that doesn't mean you should be charging your phone all night, every night.