The sort-package-json package sorts not only dependencies and devDependencies, but other keys as well. I know the original questions didn't ask about the other keys, but I think it's cool to have all keys sorted.

You can simply run:

npx sort-package-json

Example from the package page:

$ cd my-project

$ cat package.json
{
  "dependencies": {
    "sort-package-json": "1.0.0",
    "sort-object-keys": "1.0.0"
  },
  "version": "1.0.0",
  "name": "my-awesome-project"
}
 
$ npx sort-package-json
package.json is sorted!
 
$ cat package.json
{
  "name": "my-awesome-project",
  "version": "1.0.0",
  "dependencies": {
    "sort-object-keys": "1.0.0",
    "sort-package-json": "1.0.0"
  }
}

This does not remove the trailing newline like the npm-sort package mentioned by Wolfgang.

Multiple files

$ sort-package-json "my-package/package.json" "other-package/package.json"

$ sort-package-json "package.json" "packages/*/package.json"