From f6f68cb2db1cb489ddef139e269da2201c4df196 Mon Sep 17 00:00:00 2001
From: semimikoh <ejffjeosms@gmail.com>
Date: Sat, 18 Apr 2026 12:34:15 +0900
Subject: [PATCH] assert: avoid expensive diff for large values

---
 lib/internal/assert/assertion_error.js | 16 ++++++++++++++++
 test/parallel/test-assert-deep.js      | 13 +++++++++++++
 2 files changed, 29 insertions(+)

diff --git a/lib/internal/assert/assertion_error.js b/lib/internal/assert/assertion_error.js
index 5dbf1e7a341380..8fb8134ac1d6cd 100644
--- a/lib/internal/assert/assertion_error.js
+++ b/lib/internal/assert/assertion_error.js
@@ -42,6 +42,8 @@ const kReadableOperator = {
 
 const kMaxShortStringLength = 12;
 const kMaxLongStringLength = 512;
+const kMaxDiffLineCount = 1000;
+const kMaxDiffLinesToShow = 50;
 
 const kMethodsWithCustomMessageDiff = new SafeSet()
   .add('deepStrictEqual')
@@ -182,6 +184,13 @@ function isSimpleDiff(actual, inspectedActual, expected, inspectedExpected) {
   return typeof actual !== 'object' || actual === null || typeof expected !== 'object' || expected === null;
 }
 
+function getTruncatedDiffValue(lines) {
+  if (lines.length > kMaxDiffLinesToShow) {
+    return `${ArrayPrototypeJoin(ArrayPrototypeSlice(lines, 0, kMaxDiffLinesToShow), '\n')}\n...`;
+  }
+  return ArrayPrototypeJoin(lines, '\n');
+}
+
 function createErrDiff(actual, expected, operator, customMessage, diffType = 'simple') {
   operator = checkOperator(actual, expected, operator);
 
@@ -213,6 +222,13 @@ function createErrDiff(actual, expected, operator, customMessage, diffType = 'si
       message = ArrayPrototypeJoin(inspectedSplitActual, '\n');
     }
     header = '';
+  } else if (
+    diffType !== 'full' &&
+    inspectedSplitActual.length + inspectedSplitExpected.length > kMaxDiffLineCount
+  ) {
+    message = `\n${colors.green}+${colors.white} ${getTruncatedDiffValue(inspectedSplitActual)}\n` +
+              `${colors.red}-${colors.white} ${getTruncatedDiffValue(inspectedSplitExpected)}`;
+    skipped = true;
   } else {
     const checkCommaDisparity = actual != null && typeof actual === 'object';
     const diff = myersDiff(inspectedSplitActual, inspectedSplitExpected, checkCommaDisparity);
diff --git a/test/parallel/test-assert-deep.js b/test/parallel/test-assert-deep.js
index 5d856bf1f378b9..dc72f5c7654394 100644
--- a/test/parallel/test-assert-deep.js
+++ b/test/parallel/test-assert-deep.js
@@ -1180,6 +1180,19 @@ test('Strict equal with identical objects that are not identical ' +
   );
 });
 
+test('Strict equal skips line diff for very large objects', () => {
+  const buffer = Buffer.alloc(1_000);
+
+  assert.throws(
+    () => assert.strictEqual(buffer, [buffer]),
+    {
+      code: 'ERR_ASSERTION',
+      name: 'AssertionError',
+      message: /Skipped lines[\s\S]*Buffer\(1000\)/
+    }
+  );
+});
+
 test('Basic valueOf check', () => {
   const a = new String(1);
   a.valueOf = undefined;