These are some simple examples for using Slash Commands in discord.js.
discord.js doesn't have full support for slash commands yet (there's a wip pr) but you can still use the underlying api and websocket to use them.
Note that discord.js doesn't officially support using client.api, this is basically just a workaround until they fully release support.

Please read Discord's Slash Command docs since they have actual docs and details for slash commands; the code examples below are just how you can implement it using discord.js.

Note that slash commands won't show in a server unless that server has authorized it with the applications.commands oauth2 scope (not just the bot scope).