If I install a mod in Minecraft Java edition, will it erase existing worlds?

I am old and have no idea how any of this works. My son’s only Christmas request is a furniture mod for Minecraft Java edition.

If I (somehow) download and install one, will that erase his existing worlds?

I want to surprise him by building a world with furniture in because I know he’ll hop on the computer as soon as we’re done with presents on Xmas morning. I do NOT want to surprise him by inadvertently erasing all his hard work. Help!

Solution 1:

No, it should not wipe out your worlds. JUST IN CASE, HOWEVER, BACKUP YOUR WORLDS. If they do get erased, just restore the backed up worlds, and he won't be sad about losing the, because they got restored.