Failed test doesn't show diff

[sanfilippopablo]

• babel-plugin-tester version: 5.4.0
• node version: 9.8.0
• npm (or yarn) version: 5.6.0

Relevant code or config

const pluginTester = require('babel-plugin-tester')
const myPlugin = require('./plugin')

pluginTester({
  plugin: myPlugin,
  filename: __filename,
  tests: [{
    fixture: '__fixtures__/1/from.js',
    outputFixture: '__fixtures__/1/to.js'
  }],
  babelOptions: {
    babelrc: true
  }
})

What you did:
I ran jest using jest test.js and the test failed, like it should, but I was expecting the usual diff showing what it got and what it expected, but only the following output appeared. Am I doing something wrong or does it only show that message?

What happened:

 FAIL  ./test.js
  ast-transform
    ✕ 1. ast-transform (57ms)

  ● ast-transform › 1. ast-transform

    AssertionError [ERR_ASSERTION]: Output is incorrect.

      at tester (node_modules/babel-plugin-tester/dist/index.js:206:28)
      at Object.<anonymous> (node_modules/babel-plugin-tester/dist/index.js:114:13)
      at step (node_modules/babel-plugin-tester/dist/index.js:43:191)
      at node_modules/babel-plugin-tester/dist/index.js:43:361

Test Suites: 1 failed, 1 total
Tests:       1 failed, 1 total
Snapshots:   0 total
Time:        1.49s
Ran all test suites matching /test.js/i.

[kentcdodds]

Hi @sanfilippopablo!

I'm pretty sure this is a bug in jest. It should show the assert actual and expected values. Could make a simple reproduction (doesn't need to use babel-plugin-tester) that uses assert rather than expect and file an issue in Jest?

I don't think we should make changes to this project.

[sanfilippopablo]

I tried like that but I couldn't reproduce, it shows the diff just fine. The problem is when I use babel-plugin-tester. I created a minimal repo with two tests, one using pluginTester and other using assert directly. The first doesn't show a diff, but the other one does.

https://github.com/sanfilippopablo/jest-no-diff-output

[kentcdodds]

Interesting! Thanks for making that! Would you be willing to dig a little bit to figure out what's wrong?

[sanfilippopablo]

Yes! On it. Let's see what I can find.

[sanfilippopablo]

Found the issue. It's most definitely a bug in jest, not in this project. The problem is when using assert on a test whose function returns a promise.

const assert = require('assert')

// It shows the diff
test('it shows diff', () => {
  assert.equal('hello\ngoodbye', 'hello', 'hmmm')
})

// It doesn't show the diff
test('it shows diff async', async () => {
  assert.equal('hello\ngoodbye', 'hello', 'hmmm')
})

Closing this one.

[kentcdodds]

Awesome! Thanks for looking into it! I hope it can be fixed!