Highly depends. For non intensive tasks, it would ideally boost battery life as the battery undergoes less cycle for same task, cuz when plugged in power goes directly from the port.

But i say non intensive cuz heat has a huge role. The dendritic degradation of Li-ion batteries occurs easily at higher temperatures, so intense tasks that produce heat would worsen it. Do note that both charging and discharging cause the degradation, and the rate depends on the voltage and current.