implement Merge type

src/redux/features/api.types.ts

@@ -1,16 +1,14 @@
+import { Merge } from '../../utils/helpers';
+
 export namespace Evaluation {
     export type CorrectFragmentData = {
         op: 'nochange';
         word: string;
-        replace?: undefined;
-        indicator?: undefined;
-        feedback?: undefined;
     };
 
     export type SingleFragmentData = {
         op: 'addition' | 'deletion';
         word: string;
-        replace?: undefined;
         indicator: string;
         feedback: string;
     };
@@ -23,10 +21,9 @@ export namespace Evaluation {
         feedback: string;
     };
 
-    export type SentenceFragmentData =
-        | CorrectFragmentData
-        | SingleFragmentData
-        | DoubleFragmentData;
+    export type SentenceFragmentData = Merge<
+        [CorrectFragmentData, SingleFragmentData, DoubleFragmentData]
+    >;
 
     export type SentenceData = SentenceFragmentData[];
 

src/utils/helpers.ts

@@ -0,0 +1,25 @@
+/**
+ * Extracts the keys of multiple records.
+ *
+ * Usage: `Keys<[RecordA, RecordB, RecordC]>`
+ */
+type Keys<T> = T extends [infer A, ...infer R] ? keyof A | Keys<R> : never;
+
+/**
+ * Extends `T` to have at least all the keys in `Keys`.
+ * Everything not already in `T` will be optional and `undefined`.
+ */
+type Extend<T, Keys extends string | number> = T & {
+    [K in Exclude<Keys, keyof T>]?: undefined;
+};
+
+type MergeInner<T, Keys extends string | number> = T extends [infer A, ...infer R]
+    ? Extend<A, Keys> | MergeInner<R, Keys>
+    : never;
+
+/**
+ * Creates a union of multiple records.
+ * Every record is modified to contain all the keys from all records,
+ * but 'new' keys will be optional and `undefined`.
+ */
+export type Merge<T> = MergeInner<T, Keys<T>>;