Probably both. You can't use NPM (though you can use one of those online CDN's like unpkg) and the packages you can use probably don't work, as Deno doesn't support Node's API at all. It doesn't support any Node-specific packages like 'fs' or 'crypto' either.

Unless you're writing a totally new application and don't need anything (even retrospectively) from NPM, Deno probably won't work for you.

As much as I like Deno as a concept, I can't really use it. Making a totally new JavaScript runtime with new API's and no compatibility with the current largest runtime is a very ambitious goal, and I hope they succeed with it nevertheless.