assert: handle cause when cutting off stack frames

lib/internal/assert/assertion_error.js

@@ -48,6 +48,15 @@ const kMaxShortLength = 12;
 function copyError(source) {
   const target = ObjectAssign({ __proto__: ObjectGetPrototypeOf(source) }, source);
   ObjectDefineProperty(target, 'message', { __proto__: null, value: source.message });
+  if (source.cause !== undefined) {
+    let cause = source.cause;
+
+    if (cause instanceof Error) {
+      cause = copyError(cause);
+    }
+
+    ObjectDefineProperty(target, 'cause', { __proto__: null, value: cause });
+  }
   return target;
 }
 
@@ -82,8 +91,9 @@ function createErrDiff(actual, expected, operator, message = '') {
   let end = '';
   let skipped = false;
   const actualInspected = inspectValue(actual);
+  const expectedInspected = inspectValue(expected);
   const actualLines = StringPrototypeSplit(actualInspected, '\n');
-  const expectedLines = StringPrototypeSplit(inspectValue(expected), '\n');
+  const expectedLines = StringPrototypeSplit(expectedInspected, '\n');
 
   let i = 0;
   let indicator = '';
@@ -360,6 +370,7 @@ class AssertionError extends Error {
         expected = copyError(expected);
       }
 
+
       if (operator === 'deepStrictEqual' || operator === 'strictEqual') {
         super(createErrDiff(actual, expected, operator));
       } else if (operator === 'notDeepStrictEqual' ||

test/parallel/test-assert-deep-with-error.js

@@ -0,0 +1,26 @@
+'use strict';
+require('../common');
+const assert = require('assert');
+const { test } = require('node:test');
+
+const defaultStartMessage = 'Expected values to be strictly deep-equal:\n' +
+  '+ actual - expected\n' +
+  '\n';
+
+test('Handle error causes', () => {
+  assert.throws(() => {
+    assert.deepStrictEqual(new Error('a', { cause: new Error('x') }), new Error('a', { cause: new Error('y') }));
+  }, { message: defaultStartMessage + '  [Error: a] {\n' +
+    '+   [cause]: [Error: x]\n' +
+    '-   [cause]: [Error: y]\n' +
+    '  }' });
+
+  assert.throws(() => {
+    assert.deepStrictEqual(new Error('a'), new Error('a', { cause: new Error('y') }));
+  }, { message: defaultStartMessage + '+ [Error: a]\n' +
+    '- [Error: a] {\n' +
+    '-   [cause]: [Error: y]\n' +
+    '- }' });
+
+  assert.notDeepStrictEqual(new Error('a', { cause: new Error('x') }), new Error('a', { cause: new Error('y') }));
+});

test/parallel/test-assert-deep.js

@@ -611,12 +611,6 @@ test('Handle different error messages', () => {
   assertNotDeepOrStrict(err1, {}, AssertionError);
 });
 
-test.only('Handle error causes', () => {
-  // This should fail as in assert.notEqual(1, 1) (?)
-  assert.notEqual(new Error("a"), new Error("b", { cause: new Error("y") }));
-  assert.deepStrictEqual(new Error("a"), new Error("a", { cause: new Error("y") }))
-});
-
 test('Handle NaN', () => {
   assertDeepAndStrictEqual(NaN, NaN);
   assertDeepAndStrictEqual({ a: NaN }, { a: NaN });