From 4992ce29f2f931b84f91019cb7c85ee0ebe3320e Mon Sep 17 00:00:00 2001
From: Yarchik <spoko.dev@gmail.com>
Date: Wed, 17 Jun 2026 15:57:53 +0100
Subject: [PATCH] fix: preserve a trailing hole in sparse arrays

`Array.prototype.map` keeps holes, but joining the results drops a
trailing one: `[1, ,]` (length 2) was serialized as `[1,]`, which is an
array of length 1. So a sparse array ending in a hole came back shorter
after `eval`. Append a separator when the last element is a hole to keep
the length.
---
 src/array.ts      | 7 ++++++-
 src/index.spec.ts | 8 ++++++++
 2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/src/array.ts b/src/array.ts
index 5cd6e38..f129484 100644
--- a/src/array.ts
+++ b/src/array.ts
@@ -15,6 +15,11 @@ export const arrayToString: ToString = (array: any[], space, next) => {
     })
     .join(space ? ",\n" : ",");
 
+  // A trailing hole is dropped by an array literal (`[1,]` has length `1`), so
+  // append a separator to preserve the array length on the round-trip.
+  const trailingHole =
+    array.length > 0 && !(array.length - 1 in array) ? "," : "";
+
   const eol = space && values ? "\n" : "";
-  return `[${eol}${values}${eol}]`;
+  return `[${eol}${values}${trailingHole}${eol}]`;
 };
diff --git a/src/index.spec.ts b/src/index.spec.ts
index e03d411..73b657e 100644
--- a/src/index.spec.ts
+++ b/src/index.spec.ts
@@ -101,6 +101,14 @@ describe("javascript-stringify", () => {
     describe("arrays", () => {
       it("should stringify as array shorthand", test([1, 2, 3], "[1,2,3]"));
 
+      it("should preserve a trailing hole", testRoundTrip("[1,,]"));
+
+      it("should preserve multiple trailing holes", testRoundTrip("[1,,,]"));
+
+      it("should preserve a leading hole", testRoundTrip("[,1]"));
+
+      it("should preserve an interior hole", testRoundTrip("[1,,3]"));
+
       it(
         "should indent elements",
         test([{ x: 10 }], "[\n\t{\n\t\tx: 10\n\t}\n]", "\t"),