From 18ab55d9abd80d10eb908f7339720a1f3cbe0af7 Mon Sep 17 00:00:00 2001
From: Yarchik <spoko.dev@gmail.com>
Date: Thu, 23 Jul 2026 14:34:07 +0100
Subject: [PATCH] Preserve arrays under unflatten when safe and object are both
 set

`safe` is documented to make "both flatten and unflatten preserve arrays
and their contents", but with `object: true` also set, `unflatten` turns a
preserved array back into a plain object:

  const opts = { safe: true, object: true }
  unflatten(flatten({ tags: ['a', 'b', 'c'] }, opts), opts)
  // { tags: { '0': 'a', '1': 'b', '2': 'c' } }  -- no longer an array

`unflatten` never truly preserves arrays; it only rebuilds them as a side
effect of the numeric-key -> array logic, which `object: true` disables. Its
"messy objects" reduce re-flattens every container value, including a
`safe`-kept array, back into the flat keyspace, and it is then rebuilt as an
object. `safe` alone round-trips correctly because the array is rebuilt
numerically.

Pass a `safe` array through the reduce instead of re-flattening it, so it
survives unchanged regardless of `object`.
---
 index.js     | 5 ++++-
 test/test.js | 6 ++++++
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/index.js b/index.js
index 42e1ddc..9c32520 100644
--- a/index.js
+++ b/index.js
@@ -99,7 +99,10 @@ export function unflatten (target, opts) {
   target = Object.keys(target).reduce(function (result, key) {
     const type = Object.prototype.toString.call(target[key])
     const isObject = (type === '[object Object]' || type === '[object Array]')
-    if (!isObject || isEmpty(target[key])) {
+    // a `safe`-preserved array is a leaf value here, not a nested object to
+    // re-flatten; pass it through so `object: true` doesn't rebuild it as an
+    // object and break the `safe` contract of keeping arrays intact
+    if (!isObject || isEmpty(target[key]) || (opts.safe && Array.isArray(target[key]))) {
       result[key] = target[key]
       return result
     } else {
diff --git a/test/test.js b/test/test.js
index ef01c9b..954c2f2 100644
--- a/test/test.js
+++ b/test/test.js
@@ -425,6 +425,12 @@ describe('Unflatten', function () {
         bar: {}
       }), { foo: [], bar: {} })
     })
+
+    test('Should keep arrays when both safe and object are set', function () {
+      const opts = { safe: true, object: true }
+      const flat = flatten({ tags: ['red', 'green', 'blue'] }, opts)
+      assert.deepStrictEqual(unflatten(flat, opts), { tags: ['red', 'green', 'blue'] })
+    })
   })
 
   describe('.object', function () {