The Deno doc here says,

The formatter can be configured in a deno.json file. You can specify custom rules, plugins, and settings to tailor the formatting process to your needs.

but never says how. But it says the underlying formatter is Dprint, so presumably the above can be accomplished with the Dprint options documented here.

But if you add any of those to deno.json, deno fmt complains.

This needlessly precludes the use of deno fmt for anyone who doesn't like the options it hard-codes outside the tiny subset that's allowed. For example, I can't prevent the formatter from messing up my braces and flow-control statements. But Dprint offers the exact options I want:

"bracePosition": "nextLine",
"nextControlFlowPosition": "nextLine"