Fix type inference for extended i18n schemas

.changeset/spotty-eggs-sit.md

@@ -0,0 +1,5 @@
+---
+'@astrojs/starlight': patch
+---
+
+Fixes type inference for i18n strings added by extending the default schema

packages/starlight/schemas/i18n.ts

@@ -18,14 +18,21 @@ interface i18nSchemaOpts<T extends z.AnyZodObject = z.ZodObject<{}>> {
 	extend?: T;
 }
 
+const defaultI18nSchema = () =>
+	starlightI18nSchema().merge(pagefindI18nSchema()).merge(expressiveCodeI18nSchema());
+/** Type of Starlight’s default i18n schema, including extensions from Pagefind and Expressive Code. */
+type DefaultI18nSchema = ReturnType<typeof defaultI18nSchema>;
+
+/** Type that extends Starlight’s default i18n schema with an optional, user-defined schema. */
+type ExtendedSchema<T extends z.AnyZodObject> = T extends z.AnyZodObject
+	? z.ZodIntersection<DefaultI18nSchema, T>
+	: DefaultI18nSchema;
+
 /** Content collection schema for Starlight’s optional `i18n` collection. */
 export function i18nSchema<T extends z.AnyZodObject = z.ZodObject<{}>>({
 	extend = z.object({}) as T,
-}: i18nSchemaOpts<T> = {}) {
-	return starlightI18nSchema()
-		.merge(pagefindI18nSchema())
-		.merge(expressiveCodeI18nSchema())
-		.merge(extend);
+}: i18nSchemaOpts<T> = {}): ExtendedSchema<T> {
+	return defaultI18nSchema().merge(extend) as ExtendedSchema<T>;
 }
 export type i18nSchemaOutput = z.output<ReturnType<typeof i18nSchema>>;
 

packages/starlight/utils/createTranslationSystem.ts

@@ -2,8 +2,8 @@ import type { i18nSchemaOutput } from '../schemas/i18n';
 import builtinTranslations from '../translations/index';
 import type { StarlightConfig } from './user-config';
 
-export function createTranslationSystem(
-	userTranslations: Record<string, i18nSchemaOutput>,
+export function createTranslationSystem<T extends i18nSchemaOutput>(
+	userTranslations: Record<string, T>,
 	config: Pick<StarlightConfig, 'defaultLocale' | 'locales'>
 ) {
 	/** User-configured default locale. */
@@ -67,18 +67,20 @@ function localeToLang(
 	return lang || defaultLang || 'en';
 }
 
+type BuiltInStrings = (typeof builtinTranslations)['en'];
+
 /** Build a dictionary by layering preferred translation sources. */
-function buildDictionary(
-	base: (typeof builtinTranslations)[string],
-	...dictionaries: (i18nSchemaOutput | undefined)[]
-) {
+function buildDictionary<T extends Record<string, string | undefined>>(
+	base: BuiltInStrings,
+	...dictionaries: (T | BuiltInStrings | undefined)[]
+): BuiltInStrings & T {
 	const dictionary = { ...base };
 	// Iterate over alternate dictionaries to avoid overwriting preceding values with `undefined`.
 	for (const dict of dictionaries) {
 		for (const key in dict) {
-			const value = dict[key];
+			const value = dict[key as keyof typeof dict];
 			if (value) dictionary[key as keyof typeof dictionary] = value;
 		}
 	}
-	return dictionary;
+	return dictionary as BuiltInStrings & T;
 }

packages/starlight/utils/translations.ts

@@ -1,11 +1,15 @@
-import { getCollection } from 'astro:content';
+import { getCollection, type CollectionEntry, type DataCollectionKey } from 'astro:content';
 import config from 'virtual:starlight/user-config';
 import type { i18nSchemaOutput } from '../schemas/i18n';
 import { createTranslationSystem } from './createTranslationSystem';
 
+type UserI18nSchema = 'i18n' extends DataCollectionKey
+	? CollectionEntry<'i18n'>['data']
+	: i18nSchemaOutput;
+
 /** Get all translation data from the i18n collection, keyed by `id`, which matches locale. */
 async function loadTranslations() {
-	let userTranslations: Record<string, i18nSchemaOutput> = {};
+	let userTranslations: Record<string, UserI18nSchema> = {};
 	// Briefly override `console.warn()` to silence logging when a project has no i18n collection.
 	const warn = console.warn;
 	console.warn = () => {};