What's the best way to charge iPhone XS?

When to charge your iPhone or iPad - Apple Community is an excellent User Tip authored by Lawrence Finch.

Do NOT let your battery run down. It's actually bad for Lithium iON batteriers and will diminish the usefull life of the battery. Charge your battery anytime you want. What I do is use my phone during the day and plug it in to charge at night when I sleep. I have a full battery in the morning and rarely if ever need to charge during awake hours.

Partial discharges, and partial charges are fine for lithium batteries. As mentioned, avoid deep cycle discharges as they rapidly degrade the useful life span of the battery.