feat: improve support for union input types

dextertanyj · May 4, 2023 · 1dca246 · 1dca246
1 parent 3ff55da
commit 1dca246
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Showing 2 changed files with 1,754 additions and 10 deletions.
diff --git a/src/types/transformed.ts b/src/types/transformed.ts
@@ -25,7 +25,7 @@ type GetFieldType<T, Path extends string> = NormalizePath<Path> extends ""
   ? GetArrayFieldType<T, NormalizePath<Path>>
   : NormalizePath<Path> extends `${string}`
   ? GetRecordFieldType<T, NormalizePath<Path>>
-  : never;
+  : never; // Invalid Path.
 
 /**
  * Returns the type of the field defined by `Path`, resolved with respect to the record `T`.
@@ -38,14 +38,16 @@ type GetFieldType<T, Path extends string> = NormalizePath<Path> extends ""
 type GetRecordFieldType<T, Path> = T extends Record<string, unknown> | undefined | null
   ? Path extends "" // Preserves original optionality of T.
     ? T
+    : [T] extends [undefined | null] // Short circuit if T is only undefined or null.
+    ? undefined
     : Path extends keyof Defined<T>
     ? OptionalWrapper<T, Defined<T>[Path]>
     : Path extends `${infer Key extends keyof T & string}${"." | `[${number | "*"}]`}${string}`
     ? Path extends `${Key}${infer Rest extends string}`
       ? OptionalWrapper<T, GetFieldType<T[Key], NormalizePath<Rest>>>
-      : never
-    : never
-  : never;
+      : never // Invalid Path.
+    : undefined // Key is not keyof T.
+  : undefined; // T is not a Record.
 
 /**
  * Returns the type of the field defined by `Path`, resolved with respect to the array `Array`.
@@ -55,17 +57,29 @@ type GetRecordFieldType<T, Path> = T extends Record<string, unknown> | undefined
  * @typeparam `Array` - The current array type.
  * @typeparam `Path` - The path to the field.
  */
-type GetArrayFieldType<Array, Path extends string> = Array extends unknown[] | undefined | null
+type GetArrayFieldType<T, Path extends string> = T extends unknown[] | undefined | null
   ? Path extends ""
-    ? Array
+    ? T
+    : [T] extends [undefined | null] // Short circuit if T is only undefined or null.
+    ? undefined
     : Path extends `[${infer Index extends number | "*"}]${infer Next}`
-    ? Defined<Array>[number] extends infer Element
+    ? Defined<T>[number] extends infer Element
       ? GetFieldType<Element, Next> extends infer Result
-        ? WrapArrayResult<Array, Result, Index, Next>
+        ? [Result] extends [undefined]
+          ? // Element is guaranteed to not be never since T is not undefined or null,
+            // but the field cannot exist on any Element.
+            //
+            // Return never instead of undefined since the field must on elements of some
+            // other sibling T (otherwise the Path should not pass typechecking).
+            // At runtime, this T and its sibling arrays cannot be fully differentiated
+            // since optional and undefined properties of nested objects cannot be
+            // differentiated from invalid properties.
+            never
+          : WrapArrayResult<T, Result, Index, Next> // Field possibly exists on Element.
         : never
       : never
     : never
-  : never;
+  : undefined; // T is not an Array.
 
 /**
  * Returns `Result` wrapped in an array if Index is a wildcard and no other wildcards are present in `Path`.
@@ -76,7 +90,10 @@ type WrapArrayResult<Array, Result, Index, Path extends string> = Index extends
   : NormalizePath<Path> extends ""
   ? OptionalWrapper<Array, ArrayOf<Result>>
   : Contains<NormalizePath<Path>, "[*]"> extends true
-  ? OptionalWrapper<Array, Exclude<Result, undefined>>
+  ? // Merge flattened array types for results that contain union types.
+    Exclude<Result, undefined> extends (infer T)[]
+    ? OptionalWrapper<Array, ArrayOf<T>>
+    : never
   : OptionalWrapper<Array, ArrayOf<Result>>;
 
 type DeclaredArrayTransformed<T, ArrayPath extends string, Value> = GetFieldType<