Set up translation with i18next

src/TranslatedError.ts

@@ -0,0 +1,41 @@
+/*
+Copyright 2022 New Vector Ltd
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import i18n from "i18next";
+
+/**
+ * An error with messages in both English and the user's preferred language.
+ */
+// Abstract to force consumers to use the function below rather than calling the
+// constructor directly
+export abstract class TranslatedError extends Error {
+  /**
+   * The error message in the user's preferred language.
+   */
+  public readonly translatedMessage: string;
+
+  public constructor(messageKey: string, translationFn: typeof i18n.t) {
+    super(translationFn(messageKey, { lng: "en-GB" }));
+    this.translatedMessage = translationFn(messageKey);
+  }
+}
+
+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, t: typeof i18n.t) =>
+  new TranslatedErrorImpl(messageKey, t);