deepStrictEqual diff is unhelpful when prototype mismatches

Version

v20.8.0

Platform

Darwin Me.local 23.0.0 Darwin Kernel Version 23.0.0: Fri Sep 15 14:41:34 PDT 2023; root:xnu-10002.1.13~1/RELEASE_ARM64_T8103 arm64

Subsystem

No response

What steps will reproduce the bug?

Given repo.js

import { test } from "node:test";
import { deepStrictEqual, notDeepStrictEqual } from "node:assert";

class ExtendedArray extends Array {
  constructor() {
    super();
  }
}

test("deepStrictEqual should print prototype diff", () => {
  const actual = new ExtendedArray();
  actual[0] = "hello";
  const expected = ["hello"];

  notDeepStrictEqual(
    Object.getPrototypeOf(actual),
    Object.getPrototypeOf(expected)
  );

  deepStrictEqual(actual, expected);
});

Run node --test repo.js.

How often does it reproduce? Is there a required condition?

Always

What is the expected behavior? Why is that the expected behavior?

deepStrictEquals requires actual and expected to have the same prototype, but fails to show error message describing if they are not the same.

A more helpful diff would be:

$ node --test repo.js
✖ deepStrictEqual should print prototype diff (1.581625ms)
  AssertionError [ERR_ASSERTION]: Expected values to be strictly deep-equal:
  + actual - expected
  
  + ExtendedArray(1) [
  - [
      'hello'
    ]
      at TestContext.<anonymous> (file://.../repo.js:20:3)
      at Test.runInAsyncScope (node:async_hooks:206:9)
      at Test.run (node:internal/test_runner/test:631:25)
      at Test.start (node:internal/test_runner/test:542:17)
      at startSubtest (node:internal/test_runner/harness:208:17) {
    generatedMessage: true,
    code: 'ERR_ASSERTION',
    actual prototype: Array {}
    expected prototype: Object(0) [],
    operator: 'deepStrictEqual'
  }

What do you see instead?

Only the contents of the arrays are diffed.

$ node --test repo.js
✖ deepStrictEqual should print prototype diff (1.581625ms)
  AssertionError [ERR_ASSERTION]: Expected values to be strictly deep-equal:
  + actual - expected
  
  + ExtendedArray(1) [
  - [
      'hello'
    ]
      at TestContext.<anonymous> (file://.../repo.js:20:3)
      at Test.runInAsyncScope (node:async_hooks:206:9)
      at Test.run (node:internal/test_runner/test:631:25)
      at Test.start (node:internal/test_runner/test:542:17)
      at startSubtest (node:internal/test_runner/harness:208:17) {
    generatedMessage: true,
    code: 'ERR_ASSERTION',
    actual: [ 'hello' ],
    expected: [ 'hello' ],
    operator: 'deepStrictEqual'
  }

Additional information

No response

Start with the repo.js reproduction and run node --test repo.js to observe the current assertion output. Trace the node:assert deepStrictEqual failure formatting and add a regression test; done means mismatched prototypes are represented in the diff while existing array contents remain correctly reported.