Wire up typescript safety net around i18next

src/TranslatedError.ts

@@ -16,6 +16,9 @@ limitations under the License.
 
 import i18n from "i18next";
 
+import type { ParseKeys, TFunction } from "i18next/typescript/t";
+import type { DefaultNamespace, TOptions } from "i18next/typescript/options";
+
 /**
  * An error with messages in both English and the user's preferred language.
  */
@@ -27,8 +30,11 @@ export abstract class TranslatedError extends Error {
    */
   public readonly translatedMessage: string;
 
-  public constructor(messageKey: string, translationFn: typeof i18n.t) {
-    super(translationFn(messageKey, { lng: "en-GB" }));
+  public constructor(
+    messageKey: ParseKeys<DefaultNamespace, TOptions>,
+    translationFn: TFunction<DefaultNamespace>,
+  ) {
+    super(translationFn(messageKey, { lng: "en-GB" } as TOptions));
     this.translatedMessage = translationFn(messageKey);
   }
 }
@@ -38,6 +44,6 @@ class TranslatedErrorImpl extends TranslatedError {}
 // i18next-parser can't detect calls to a constructor, so we expose a bare
 // function instead
 export const translatedError = (
-  messageKey: string,
+  messageKey: ParseKeys<DefaultNamespace, TOptions>,
   t: typeof i18n.t,
 ): TranslatedError => new TranslatedErrorImpl(messageKey, t);