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 1/2] 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;

From f41f6cf72372df096af26466b13714f0268e405d Mon Sep 17 00:00:00 2001
From: semimikoh <ejffjeosms@gmail.com>
Date: Mon, 20 Apr 2026 15:40:03 +0900
Subject: [PATCH 2/2] assert: address large diff review feedback

---
 lib/internal/assert/assertion_error.js | 24 +++++++++++++++---------
 test/parallel/test-assert-deep.js      | 15 +++++++++++++++
 2 files changed, 30 insertions(+), 9 deletions(-)

diff --git a/lib/internal/assert/assertion_error.js b/lib/internal/assert/assertion_error.js
index 8fb8134ac1d6cd..1561f7804efbb1 100644
--- a/lib/internal/assert/assertion_error.js
+++ b/lib/internal/assert/assertion_error.js
@@ -191,6 +191,12 @@ function getTruncatedDiffValue(lines) {
   return ArrayPrototypeJoin(lines, '\n');
 }
 
+function hasLargeRootMismatch(inspectedActual, inspectedExpected, diffType) {
+  return diffType !== 'full' &&
+         inspectedActual.length + inspectedExpected.length > kMaxDiffLineCount &&
+         inspectedActual[0] !== inspectedExpected[0];
+}
+
 function createErrDiff(actual, expected, operator, customMessage, diffType = 'simple') {
   operator = checkOperator(actual, expected, operator);
 
@@ -222,11 +228,11 @@ 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} ${getTruncatedDiffValue(inspectedSplitActual)}\n` +
+  } else if (hasLargeRootMismatch(inspectedSplitActual, inspectedSplitExpected, diffType)) {
+    const actualPrefix = operator === 'partialDeepStrictEqual' ?
+      `${colors.gray}${colors.hasColors ? ' ' : '+'}` :
+      `${colors.green}+${colors.white}`;
+    message = `\n${actualPrefix} ${getTruncatedDiffValue(inspectedSplitActual)}\n` +
               `${colors.red}-${colors.white} ${getTruncatedDiffValue(inspectedSplitExpected)}`;
     skipped = true;
   } else {
@@ -236,15 +242,15 @@ function createErrDiff(actual, expected, operator, customMessage, diffType = 'si
     const myersDiffMessage = printMyersDiff(diff, operator);
     message = myersDiffMessage.message;
 
-    if (operator === 'partialDeepStrictEqual') {
-      header = `${colors.gray}${colors.hasColors ? '' : '+ '}actual${colors.white} ${colors.red}- expected${colors.white}`;
-    }
-
     if (myersDiffMessage.skipped) {
       skipped = true;
     }
   }
 
+  if (operator === 'partialDeepStrictEqual') {
+    header = `${colors.gray}${colors.hasColors ? '' : '+ '}actual${colors.white} ${colors.red}- expected${colors.white}`;
+  }
+
   const headerMessage = `${getErrorMessage(operator, customMessage)}\n${header}`;
   const skippedMessage = skipped ? '\n... Skipped lines' : '';
 
diff --git a/test/parallel/test-assert-deep.js b/test/parallel/test-assert-deep.js
index dc72f5c7654394..28cfb31e368899 100644
--- a/test/parallel/test-assert-deep.js
+++ b/test/parallel/test-assert-deep.js
@@ -1193,6 +1193,21 @@ test('Strict equal skips line diff for very large objects', () => {
   );
 });
 
+test('Deep strict equal preserves line diff for large values with the same root', () => {
+  const actual = new Array(1_000).fill(0);
+  const expected = new Array(1_000).fill(0);
+  expected[10] = 1;
+
+  assert.throws(
+    () => assert.deepStrictEqual(actual, expected),
+    {
+      code: 'ERR_ASSERTION',
+      name: 'AssertionError',
+      message: /- {3}1/
+    }
+  );
+});
+
 test('Basic valueOf check', () => {
   const a = new String(1);
   a.valueOf = undefined;