feat: support custom jiti importers

src/index.ts

@@ -9,6 +9,7 @@ import type {
   LoadConfigResult,
 } from './types.js';
 
+export { setJitiLoader } from './jiti.js';
 export { withConfigMeta } from './meta.js';
 export type {
   ConfigDefinition,

src/jiti.ts

@@ -2,7 +2,18 @@ import { cachedImport } from './cachedImport.js';
 import { getConfigExport } from './helpers.js';
 import type { ConfigDefinition, LoadedConfig } from './types.js';
 
-const getJiti = cachedImport(() => import('jiti'));
+// Keep the injection API independent of the optional jiti package's types.
+type JitiLoader = () => Promise<{ createJiti: unknown }>;
+
+let getJiti: JitiLoader = cachedImport(() => import('jiti'));
+
+/**
+ * Set the lazy jiti importer for this module instance before loading configs.
+ * Replaces the cached import without invoking the new importer.
+ */
+export const setJitiLoader = (loader: JitiLoader): void => {
+  getJiti = cachedImport(loader);
+};
 
 export const loadWithJiti = async <Config, Params extends unknown[]>(
   configPath: string,
@@ -12,7 +23,7 @@ export const loadWithJiti = async <Config, Params extends unknown[]>(
   let createJiti: (typeof import('jiti'))['createJiti'];
 
   try {
-    ({ createJiti } = await getJiti());
+    ({ createJiti } = (await getJiti()) as typeof import('jiti'));
   } catch (error) {
     throw new Error(
       'The "jiti" package is required to load this config. Install it with your package manager.',

tests/loader-jiti/injection.test.ts

@@ -0,0 +1,24 @@
+import { afterEach, expect, rstest, test } from 'rstack/test';
+import { loadConfig, setJitiLoader } from '../../src/index';
+
+afterEach(() => {
+  setJitiLoader(() => import('jiti'));
+});
+
+test('lazily imports injected jiti once for concurrent configs', async () => {
+  const importer = rstest.fn(() => import('jiti'));
+  setJitiLoader(importer);
+  expect(importer).not.toHaveBeenCalled();
+
+  const options = {
+    cwd: import.meta.dirname,
+    path: 'demo.config.ts',
+    loader: 'jiti',
+  } as const;
+  const [result] = await Promise.all([
+    loadConfig(options),
+    loadConfig(options),
+  ]);
+  expect(result.content).toEqual({ loader: 'jiti' });
+  expect(importer).toHaveBeenCalledTimes(1);
+});