From 567d94e220533de9206c821a168ccca8ee244acf Mon Sep 17 00:00:00 2001
From: youdie006 <xncb135@korea.ac.kr>
Date: Wed, 9 Sep 2026 12:19:22 +0900
Subject: [PATCH 1/2] fix(csv-stringify): do not alter the source record when
 columns is set

When a record is an array and the `columns` option is shorter than that
array, the stringifier called `chunk.splice(columns.length)`, which
truncates the array owned by the caller instead of only limiting what is
written. Bound the cast loop instead, leaving the input untouched. The
object branch already reads through `get()` into a fresh local record.

Assisted-by: Claude Code:claude-opus-5
---
 packages/csv-stringify/lib/api/index.js       |  8 +++----
 packages/csv-stringify/test/option.columns.ts | 23 +++++++++++++++++++
 2 files changed, 27 insertions(+), 4 deletions(-)

diff --git a/packages/csv-stringify/lib/api/index.js b/packages/csv-stringify/lib/api/index.js
index 821d096a..2c5bc4bb 100644
--- a/packages/csv-stringify/lib/api/index.js
+++ b/packages/csv-stringify/lib/api/index.js
@@ -113,11 +113,11 @@ const stringifier = function (options, state, info) {
       if (Array.isArray(chunk)) {
         // We are getting an array but the user has specified output columns. In
         // this case, we respect the columns indexes
-        if (columns) {
-          chunk.splice(columns.length);
-        }
+        const length = columns
+          ? Math.min(chunk.length, columns.length)
+          : chunk.length;
         // Cast record elements
-        for (let i = 0; i < chunk.length; i++) {
+        for (let i = 0; i < length; i++) {
           const field = chunk[i];
           const [err, value] = this.__cast(field, {
             index: i,
diff --git a/packages/csv-stringify/test/option.columns.ts b/packages/csv-stringify/test/option.columns.ts
index 37bf8fd4..c6d1fd1a 100644
--- a/packages/csv-stringify/test/option.columns.ts
+++ b/packages/csv-stringify/test/option.columns.ts
@@ -128,6 +128,29 @@ describe("Option `columns`", function () {
       );
     });
 
+    it("is an array, should not be altered", function (next) {
+      const records = [
+        ["20322051544", "1979", "8.8017226E7"],
+        ["28392898392", "1974", "8.8392926E7"],
+      ];
+      stringify(
+        records,
+        {
+          columns: ["FIELD_1", "FIELD_2"],
+        },
+        (err, data) => {
+          if (!err) {
+            data.should.eql("20322051544,1979\n28392898392,1974\n");
+            records.should.eql([
+              ["20322051544", "1979", "8.8017226E7"],
+              ["28392898392", "1974", "8.8392926E7"],
+            ]);
+          }
+          next(err);
+        },
+      );
+    });
+
     it("is a readable stream", function (next) {
       const ws = stringify(
         {

From 2fb399d259873ac9d903a866b2857a41e8a74b00 Mon Sep 17 00:00:00 2001
From: David Worms <david@adaltas.com>
Date: Wed, 23 Sep 2026 09:59:10 +0200
Subject: [PATCH 2/2] test(csv-stringify): rename vars

---
 packages/csv-stringify/test/option.columns.ts | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/packages/csv-stringify/test/option.columns.ts b/packages/csv-stringify/test/option.columns.ts
index c6d1fd1a..be65b572 100644
--- a/packages/csv-stringify/test/option.columns.ts
+++ b/packages/csv-stringify/test/option.columns.ts
@@ -128,10 +128,10 @@ describe("Option `columns`", function () {
       );
     });
 
-    it("is an array, should not be altered", function (next) {
+    it("input array is not altered", function (next) {
       const records = [
-        ["20322051544", "1979", "8.8017226E7"],
-        ["28392898392", "1974", "8.8392926E7"],
+        ["a", "b", "c"],
+        ["d", "e", "f"],
       ];
       stringify(
         records,
@@ -140,10 +140,10 @@ describe("Option `columns`", function () {
         },
         (err, data) => {
           if (!err) {
-            data.should.eql("20322051544,1979\n28392898392,1974\n");
+            data.should.eql("a,b\nd,e\n");
             records.should.eql([
-              ["20322051544", "1979", "8.8017226E7"],
-              ["28392898392", "1974", "8.8392926E7"],
+              ["a", "b", "c"],
+              ["d", "e", "f"],
             ]);
           }
           next(err);