Prefer type imports in `import/no-duplicates`

[Mrazator]

It's possible to auto-fix detected duplicated import types as inline types with import/no-duplicates ("prefer-inline": true), but it's not possible to keep type imports as they are.

Therefore, import/no-duplicate should be extended to prefer type imports when fixing duplicates

❌ Invalid ["error", {"prefer-type-imports": true}]

import { AValue, type AType, type BType } from './mama-mia'

import { CValue, type CType } from './papa-mia'

✅ Valid with ["error", {"prefer-type-imports": true}]

import { AValue, } from './mama-mia'
import type { AType , BType } from './mama-mia'

import { CValue } from './papa-mia'
import type { CType } from './papa-mia'

[dwelle]

import { AValue, } from './mama-mia'
import type { AType , BType } from './mama-mia'

import { CValue } from './papa-mia'
import type { CType } from './papa-mia'

IMO this is fine, too.

[ljharb]

I'm confused, why would you want the two separate import statements?

[Mrazator]

Multiple reasons, in general they serve different purpose and have different characteristics.

Regular imports influence both build time (bundlers) and runtime, so we have to be careful about those. It matters what you import from where, while for types it does not matter that much (i.e. circular dependencies are fine).

Mixing the two in one import statement could lead to accidental runtime imports, which could cause headaches later on (i.e. circular dependencies, browser-only code included in server-only bundles, unnecessary large browser / server bundles, etc.).

Separating the imports also improves overall readability, so it's immediately obvious which abstractions (types) your module depends on, as you don't have to find it somewhere inside a regular import.

Is also allows us to specify specific rules for regular imports (like package A can't import from package B), while tolerating type imports e.g.:

 "@typescript-eslint/no-restricted-imports": [
  "error",
  {
    "patterns": [
      {
        "group": ["../../excalidraw", "../../../packages/excalidraw", "@excalidraw/excalidraw"],
        "message": "Do not import from '@excalidraw/excalidraw' package anything but types, as this package must be independent.",
        "allowTypeImports": true
      }
    ]
  }
]

[ljharb]

and if you set prefer-inline to false, what happens?