Show diff if test fails with an assertion that has a message

[remcohaszing]

What is the problem this feature will solve?

Let’s say we have the following test:

import assert from 'node:assert/strict'
import {test} from 'node:test'

test('test', () => {
  assert.deepEqual({foo: 1}, {bar: 2})
})

This yields:

✖ test (2.74485ms)
  AssertionError [ERR_ASSERTION]: Expected values to be strictly deep-equal:
  + actual - expected
  
    {
  +   foo: 1
  -   bar: 2
    }
      at TestContext.<anonymous> (file:///tmp/test.js:5:10)
      at Test.runInAsyncScope (node:async_hooks:203:9)
      at Test.run (node:internal/test_runner/test:550:25)
      at Test.start (node:internal/test_runner/test:466:17)
      at startSubtest (node:internal/test_runner/harness:203:17) {
    generatedMessage: true,
    code: 'ERR_ASSERTION',
    actual: [Object],
    expected: [Object],
    operator: 'deepStrictEqual'
  }

ℹ tests 1
ℹ suites 0
ℹ pass 0
ℹ fail 1
ℹ cancelled 0
ℹ skipped 0
ℹ todo 0
ℹ duration_ms 9.405037

✖ failing tests:

✖ test (2.74485ms)
  AssertionError [ERR_ASSERTION]: Expected values to be strictly deep-equal:
  + actual - expected
  
    {
  +   foo: 1
  -   bar: 2
    }
      at TestContext.<anonymous> (file:///tmp/test.js:5:10)
      at Test.runInAsyncScope (node:async_hooks:203:9)
      at Test.run (node:internal/test_runner/test:550:25)
      at Test.start (node:internal/test_runner/test:466:17)
      at startSubtest (node:internal/test_runner/harness:203:17) {
    generatedMessage: true,
    code: 'ERR_ASSERTION',
    actual: [Object],
    expected: [Object],
    operator: 'deepStrictEqual'
  }

Now we want to provide some more context for the assertion, so we add a message.

import assert from 'node:assert/strict'
import {test} from 'node:test'

test('test', () => {
  assert.deepEqual({foo: 1}, {bar: 2}, 'objects should be equal')
})

Now this yields (the diff is gone):

✖ test (1.631264ms)
  AssertionError [ERR_ASSERTION]: objects should be equal
      at TestContext.<anonymous> (file:///tmp/test.js:5:10)
      at Test.runInAsyncScope (node:async_hooks:203:9)
      at Test.run (node:internal/test_runner/test:550:25)
      at Test.start (node:internal/test_runner/test:466:17)
      at startSubtest (node:internal/test_runner/harness:203:17) {
    generatedMessage: false,
    code: 'ERR_ASSERTION',
    actual: [Object],
    expected: [Object],
    operator: 'deepStrictEqual'
  }

ℹ tests 1
ℹ suites 0
ℹ pass 0
ℹ fail 1
ℹ cancelled 0
ℹ skipped 0
ℹ todo 0
ℹ duration_ms 9.309917

✖ failing tests:

✖ test (1.631264ms)
  AssertionError [ERR_ASSERTION]: objects should be equal
      at TestContext.<anonymous> (file:///tmp/test.js:5:10)
      at Test.runInAsyncScope (node:async_hooks:203:9)
      at Test.run (node:internal/test_runner/test:550:25)
      at Test.start (node:internal/test_runner/test:466:17)
      at startSubtest (node:internal/test_runner/harness:203:17) {
    generatedMessage: false,
    code: 'ERR_ASSERTION',
    actual: [Object],
    expected: [Object],
    operator: 'deepStrictEqual'
  }

Although the intent of adding an assertion message is good, IMO the former is much more useful.

What is the feature you are proposing to solve the problem?

Show both the custom message and the diff. I.e.

✖ test (2.74485ms)
  AssertionError [ERR_ASSERTION]: objects should be equal
  + actual - expected
  
    {
  +   foo: 1
  -   bar: 2
    }
      at TestContext.<anonymous> (file:///tmp/test.js:5:10)
      at Test.runInAsyncScope (node:async_hooks:203:9)
      at Test.run (node:internal/test_runner/test:550:25)
      at Test.start (node:internal/test_runner/test:466:17)
      at startSubtest (node:internal/test_runner/harness:203:17) {
    generatedMessage: true,
    code: 'ERR_ASSERTION',
    actual: [Object],
    expected: [Object],
    operator: 'deepStrictEqual'
  }

ℹ tests 1
ℹ suites 0
ℹ pass 0
ℹ fail 1
ℹ cancelled 0
ℹ skipped 0
ℹ todo 0
ℹ duration_ms 9.405037

✖ failing tests:

✖ test (2.74485ms)
  AssertionError [ERR_ASSERTION]: objects should be equal
  + actual - expected
  
    {
  +   foo: 1
  -   bar: 2
    }
      at TestContext.<anonymous> (file:///tmp/test.js:5:10)
      at Test.runInAsyncScope (node:async_hooks:203:9)
      at Test.run (node:internal/test_runner/test:550:25)
      at Test.start (node:internal/test_runner/test:466:17)
      at startSubtest (node:internal/test_runner/harness:203:17) {
    generatedMessage: true,
    code: 'ERR_ASSERTION',
    actual: [Object],
    expected: [Object],
    operator: 'deepStrictEqual'
  }

What alternatives have you considered?

None

[AdityaPimpalkar]

Hi @MoLow, I would like to work on this issue and submit a pull request.

[MoLow]

@AdityaPimpalkar if you have a solution, you should open a PR. i'd be happy to review it

[github-actions]

There has been no activity on this feature request for 5 months and it is unlikely to be implemented. It will be closed 6 months after the last non-automated comment.

For more information on how the project manages feature requests, please consult the feature request management document.

[remcohaszing]

There is an open PR to resolve this awaiting code review.

[github-actions]

There has been no activity on this feature request for 5 months. To help maintain relevant open issues, please add the never-stale Mark issue so that it is never considered stale label or close this issue if it should be closed. If not, the issue will be automatically closed 6 months after the last non-automated comment.
For more information on how the project manages feature requests, please consult the feature request management document.

[remcohaszing]

The PR is still awaiting review