How can I read rows on JavaScript? I'm not good at Javascript. I don't think this might help, but I will write my source code down.

const username = async () => {
    return await supabase.from('users').select('username').eq('id', user?.id)
}

Yes that code should read the row, but you don't want to be allowing users to pass that

user.id

down, you should rather use a row level security policy to get things like this working. Also please take a look at the Supabase guides to learn a bit more about how to use the JavaScript library, the guides for JavaScript are really good.

Thanks for your advice.