From 1c8b13295a700ef21b5c9801a6b1b1f78fd287e5 Mon Sep 17 00:00:00 2001
From: Yarchik <spoko.dev@gmail.com>
Date: Mon, 22 Jun 2026 20:13:50 +0100
Subject: [PATCH] fix: keep an empty-string key on unflatten instead of
 renaming it to 0

`getkey` coerces a key to a number when it is numeric, guarding only
against NaN. But `Number('')` is `0` (not NaN), so a top-level empty-string
key was turned into the index 0, breaking the documented round-trip:

    unflatten(flatten({ '': 1, x: 2 }))   // { '0': 1, x: 2 }, expected { '': 1, x: 2 }
    unflatten(flatten({ a: { '': 1 } }))  // { a: [1] }

flatten preserves `''` losslessly, so only unflatten corrupted it. Return
the empty string as-is before the numeric coercion. Added a round-trip test.
---
 index.js     | 4 ++++
 test/test.js | 6 ++++++
 2 files changed, 10 insertions(+)

diff --git a/index.js b/index.js
index 42e1ddc..d86a68f 100644
--- a/index.js
+++ b/index.js
@@ -63,6 +63,10 @@ export function unflatten (target, opts) {
   // safely ensure that the key is
   // an integer.
   function getkey (key) {
+    // `Number('')` is 0 (not NaN), so an empty-string key would be coerced to
+    // the numeric index 0 and renamed; keep it as-is.
+    if (key === '') return key
+
     const parsedKey = Number(key)
 
     return (
diff --git a/test/test.js b/test/test.js
index ef01c9b..6c74eaf 100644
--- a/test/test.js
+++ b/test/test.js
@@ -566,6 +566,12 @@ describe('Arrays', function () {
     }))
   })
 
+  test('Should keep an empty-string key instead of renaming it to 0', function () {
+    assert.deepStrictEqual({ '': 1, x: 2 }, unflatten({ '': 1, x: 2 }))
+    assert.deepStrictEqual({ a: { '': 1 } }, unflatten({ 'a.': 1 }))
+    assert.deepStrictEqual({ '': 1, x: 2 }, unflatten(flatten({ '': 1, x: 2 })))
+  })
+
   test('Array typed objects should be restored by unflatten', function () {
     assert.strictEqual(
       Object.prototype.toString.call(['foo', 'bar'])