feat: make ts-node an optional dependency

Author: Codex-

lib/loader.ts

@@ -2,20 +2,34 @@ import type { Loader } from "cosmiconfig";
 import type { RegisterOptions, Service } from "ts-node";
 
 import { TypeScriptCompileError } from "./typescript-compile-error";
+import { instanceOfNodeError } from "./util";
 
 export function TypeScriptLoader(options?: RegisterOptions): Loader {
   let tsNodeInstance: Service;
 
   return (path: string, content: string) => {
-    try {
-      if (!tsNodeInstance) {
+    if (!tsNodeInstance) {
+      try {
         const { register } = require("ts-node") as typeof import("ts-node");
         tsNodeInstance = register({
           ...options,
           compilerOptions: { module: "commonjs" },
         });
+      } catch (error) {
+        if (
+          instanceOfNodeError(error) &&
+          error.code === "ERR_MODULE_NOT_FOUND"
+        ) {
+          throw new Error(
+            "cosmiconfig-typescript-loader: 'ts-node' is required for loading TypeScript cosmiconfig configuration files." +
+              `Make sure it is installed\nError: ${error.message}`
+          );
+        }
+        throw error;
       }
+    }
 
+    try {
       // cosmiconfig requires the transpiled configuration to be CJS
       tsNodeInstance.compile(content, path);
       const result = require(path);

lib/util.ts

@@ -0,0 +1,9 @@
+export function instanceOfNodeError(
+  error: unknown
+): error is NodeJS.ErrnoException {
+  return (
+    error instanceof Error &&
+    // https://github.com/DefinitelyTyped/DefinitelyTyped/commit/cddd0b7aab18761214d26a0c7012cf45de5285a9
+    (error as Record<string, any>).code !== undefined
+  );
+}

package.json

@@ -36,9 +36,11 @@
   "peerDependencies": {
     "@types/node": "*",
     "cosmiconfig": ">=7",
-    "ts-node": ">=10",
     "typescript": ">=4"
   },
+  "optionalDependencies": {
+    "ts-node": ">=10"
+  },
   "devDependencies": {
     "@swc/core": "^1.3.44",
     "@swc/jest": "^0.2.24",