fix: use type-only imports whenever possible (#460)

This PR introduces two fundamental ESLint rules:

```text
@typescript-eslint/no-import-type-side-effects
@typescript-eslint/consistent-type-imports
```

Which help avoiding situations where `tsc` keeps empty imports.

Fixes #459

Author: lppedd

eslint.config.mjs

@@ -13,6 +13,8 @@ export default [
     rules: {
       "@typescript-eslint/no-empty-object-type": ["error", { allowInterfaces: "with-single-extends" }],
       "@typescript-eslint/no-namespace": ["error", { allowDeclarations: true }],
+      "@typescript-eslint/no-import-type-side-effects": "error",
+      "@typescript-eslint/consistent-type-imports": ["error", { fixStyle: "inline-type-imports" }],
       "no-empty": ["error", { allowEmptyCatch: true }],
       "prefer-const": ["error", { destructuring: "all" }],
     },

src/plugins/nx.ts

@@ -1,7 +1,7 @@
 // See nx-transformer-plugin.ts
 // https://github.com/nrwl/nx/blob/229f71ef1758ee625869aaa6fa6355dc3284fa5b/packages/js/src/utils/typescript/types.ts#L19-L32
 // https://github.com/nrwl/nx/blob/master/packages/js/src/utils/typescript/load-ts-transformers.ts
-import ts from "typescript";
+import type ts from "typescript";
 
 import transformer from "../transformer.ts";
 

src/types.ts

@@ -1,6 +1,7 @@
-import { Minimatch } from "minimatch";
-import { type PluginConfig } from "ts-patch";
-import ts, { type CompilerOptions, type EmitHost, type Pattern, type SourceFile } from "typescript";
+import type { Minimatch } from "minimatch";
+import type { PluginConfig } from "ts-patch";
+import type ts from "typescript";
+import type { CompilerOptions, EmitHost, Pattern, SourceFile } from "typescript";
 
 /* ****************************************************************************************************************** */
 // region: TS Types

src/utils/resolve-module-name.ts

@@ -2,7 +2,7 @@ import * as path from "node:path";
 
 import { removeFileExtension, removeSuffix, type ResolvedModuleFull, type SourceFile } from "typescript";
 
-import { type VisitorContext } from "../types.ts";
+import type { VisitorContext } from "../types.ts";
 import { isBaseDir, isURL, maybeAddRelativeLocalPrefix } from "./general-utils.ts";
 import { getRelativePath } from "./get-relative-path.ts";
 import { getOutputDirForSourceFile } from "./ts-helpers.ts";

src/utils/resolve-path-update-node.ts

@@ -1,6 +1,6 @@
-import ts from "typescript";
+import type ts from "typescript";
 
-import { type VisitorContext } from "../types.ts";
+import type { VisitorContext } from "../types.ts";
 import { isURL, maybeAddRelativeLocalPrefix } from "./general-utils.ts";
 import { resolveModuleName } from "./resolve-module-name.ts";
 import { isModulePathsMatch } from "./ts-helpers.ts";

src/utils/ts-helpers.ts

@@ -1,9 +1,10 @@
 import path from "node:path";
 
 import type { REGISTER_INSTANCE } from "ts-node";
-import ts, { type GetCanonicalFileName, type SourceFile } from "typescript";
+import type ts from "typescript";
+import type { GetCanonicalFileName, SourceFile } from "typescript";
 
-import { type VisitorContext } from "../types.ts";
+import type { VisitorContext } from "../types.ts";
 
 /* ****************************************************************************************************************** */
 // region: TS Helpers

src/visitor.ts

@@ -1,6 +1,6 @@
-import ts from "typescript";
+import type ts from "typescript";
 
-import { type VisitorContext } from "./types.ts";
+import type { VisitorContext } from "./types.ts";
 import { elideImportOrExportDeclaration, resolvePathAndUpdateNode } from "./utils/index.ts";
 
 const isAsyncImport = ({ tsInstance }: VisitorContext, node: ts.Node): node is ts.CallExpression =>

test/tests/transformer/general.test.ts

@@ -2,7 +2,7 @@
 import * as path from "node:path";
 import { before, describe, test } from "node:test";
 
-import { projectsPaths, ts, tsModules } from "../../config.ts";
+import { projectsPaths, type ts, tsModules } from "../../config.ts";
 import { createTsProgram, getEmitResultFromProgram, type EmittedFiles } from "../../utils/index.ts";
 
 /* ****************************************************************************************************************** *

test/tests/transformer/specific.test.ts

@@ -3,9 +3,9 @@ import assert from "node:assert";
 import * as path from "node:path";
 import { before, describe, test } from "node:test";
 
-import TS from "typescript";
+import type TS from "typescript";
 
-import { type TsTransformPathsConfig } from "typescript-transform-paths";
+import type { TsTransformPathsConfig } from "typescript-transform-paths";
 
 import { projectsPaths, ts, tsModules } from "../../config.ts";
 import {

test/utils/helpers.ts

@@ -1,6 +1,6 @@
 import fs from "node:fs";
 
-import ts from "typescript";
+import type ts from "typescript";
 import { default as tstpTransform, type TsTransformPathsConfig } from "typescript-transform-paths";
 
 import * as config from "../config.ts";