I want to factory reset iphone8 without computer

You never could factory reset an iPhone without a computer. It has always required that. If you are asking about just a reset, that is still possible with the iPhone 8, however it is called a Force Restart. See the instructions here, Force restart iPhone - Apple Support

However, if you have forgotten the passcode like Lyssa is inquiring, or the iPhone is disabled, then you are going to have to restore the device, which requires the instructions here, If you forgot your iPhone passcode - Apple Support and a computer.

If you can access it, you can go to Settings>General>Resets>Erase All Content and settings.

Make sure you sign out of your iCloud account first.

Do you know the passcode for the phone?

What, exactly, are you trying to do?

~Lyssa