Revert previous assertion as it introduced a regression

giggo1604 · giggo1604 · commit 009229d4d767 · 2025-04-30T11:14:13.000+02:00
It seems reasonable to assert that keys shouldn't be undefined,
since keys are supposed to be of type PropertyKey (string | number | symbol).
But in practice, JavaScript allows undefined as a key by implicitly
converting it to the string "undefined".

This caused a regression specifically in the `groupBy` function, where the
grouper callback can return undefined for some items. That behavior was
previously supported and is now broken by the assertion.

Long-term, it probably makes sense to tighten this up and only allow
real PropertyKeys, but that’ll need a more careful cleanup.
diff --git a/__tests__/groupBy.ts b/__tests__/groupBy.ts
@@ -75,6 +75,13 @@ describe('groupBy', () => {
     expect(group.toJS()).toEqual({ odd: [1, 3, 5], even: [2, 4, 6] });
   });
 
+  it('allows `undefined` as a key', () => {
+    const group = Seq([1, 2, 3, 4, 5, 6]).groupBy((x) =>
+      x % 2 ? undefined : 'even'
+    );
+    expect(group.toJS()).toEqual({ undefined: [1, 3, 5], even: [2, 4, 6] });
+  });
+
   it('groups indexed sequences, maintaining indicies when keyed sequences', () => {
     const group = Seq([1, 2, 3, 4, 5, 6]).groupBy((x) => x % 2);
 
diff --git a/src/functional/updateIn.ts b/src/functional/updateIn.ts
@@ -180,13 +180,11 @@ function updateInDeeply<
   }
   const key = keyPath[i];
 
-  if (typeof key === 'undefined') {
-    throw new TypeError(
-      'Index can not be undefined in updateIn(). This should not happen'
-    );
-  }
+  const nextExisting = wasNotSet
+    ? NOT_SET
+    : // @ts-expect-error key might be undefined which is not allowed in the type but works in practice
+      get(existing, key, NOT_SET);
 
-  const nextExisting = wasNotSet ? NOT_SET : get(existing, key, NOT_SET);
   const nextUpdated = updateInDeeply(
     nextExisting === NOT_SET ? inImmutable : isImmutable(nextExisting),
     // @ts-expect-error mixed type
@@ -196,10 +194,12 @@ function updateInDeeply<
     notSetValue,
     updater
   );
+
   return nextUpdated === nextExisting
     ? existing
     : nextUpdated === NOT_SET
-      ? remove(existing, key)
+      ? // @ts-expect-error key might be undefined which is not allowed in the type but works in practice
+        remove(existing, key)
       : set(
           wasNotSet ? (inImmutable ? emptyMap() : {}) : existing,
           key,
