How do I decide whether @types/* goes into `dependencies` or `devDependencies`?

Let's say you're developing a package "A" that have @types/some-module package in devDependencies. For some reason you're exporting the type from @types/some-module:

import { SomeType } from 'some-module';

export default class APackageClass {
  constructor(private config: SomeType) {
    // …       
  }
}

Right now TypeScript consumers of package "A" are unable to guess what SomeType is, since devDependencies of package "A" are not installed.

In that particular case you need to place @types/* package with regular dependencies. For other cases devDependencies are good enough.

If you're just generating a bundle there may be no need to make the distinction between dependencies and devDependencies. This feature of npm is generally useful when publishing a package that can be used by others and you don't want to spam them with redundant dependencies.

There may be other use cases where splitting dependencies can be helpful but unless you have an express need for this, then my advice is to just pick either one and place everything there. It's not difficult to split them afterwards if the need should arise.

A well-known example of this practice IRL is create-react-app, by default the un-ejected boilerplate it creates places everything in dependencies, see this thread and this answer

In the particular case of deploying a Node.js application to production, one wants to install only the dependencies needed to run the application.

npm install --production or

npm ci --production or

yarn --production

In that case, the types should be in the devDependencies, to keep them from bloating the installation.

(To avoid misunderstandings, the --production option must not be used on machines where the application is built, otherwise the TypeScript compiler will complain.)

Remarks: I'm aware this was mentioned in a comment by Brad Wilson to another answer. This point seems worthy to be an answer, though.

Other answers made great sense, but I'm gonna add that a peerDep's type declaration package should also be placed in dependencies instead of peerDependencies.

Assume that b is a plugin of a. And c uses a and b.

Why shouldn't @types/a be placed in b's peerDependencies?

If b's package.json is like:

{
  "peerDependencies": {
    "a": "1.5.x"
    "@types/a": "1.4.x"
  }
}

c may use only interfaces defined in @types/a@1.2.x but c is forced to install @types/a@1.4.x.

Furthermore, c may be a regular javascript package rather than typescript package, but c is also forced to install @types/a@1.4.x.

So, the correct package.json of b should be like:

{
  "peerDependencies": {
    "a": "1.5.2"
  },
  "dependencies": {
    "@types/a": "1.4.x"
  }
}