Prerequisites

• Checked that your issue hasn't already been filed by cross-referencing issues with the faq label
• Checked next-gen ES issues and syntax problems by using the same environment and/or transpiler configuration without Mocha to ensure it isn't just a feature that actually isn't supported in the environment in question or a bug in your code.
• 'Smoke tested' the code to be tested by running it outside the real test suite to get a better sense of whether the problem is in the code under test, your usage of Mocha, or Mocha itself
• Ensured that there is no discrepancy between the locally and globally installed versions of Mocha. You can find them with: node node_modules/.bin/mocha --version(Local) and mocha --version(Global). We recommend that you not install Mocha globally.

Description

Due to how mocha generates the diffs of action vs expected, the results can be very confusing and misleading. There's a big difference between an array that contains undefined and the value undefined — that is, [undefined] vs undefined.

I originally thought this was an issue in Chai, but it turns out to be in Mocha:

• expect().to.deep.equal fails to handle arrays vs undefined chaijs/chai#1356
• https://github.com/mochajs/mocha/blob/v8.0.1/lib/utils.js#L307

Steps to Reproduce

Given an assertion of:

expect({
  scopes: []
}).to.deep.equal({
  scopes: undefined
})

Expected behavior: [What you expect to happen]

Output should be:

  -  "scopes": []
  +  "scopes": undefined

Actual behavior: [What actually happens]

  -  "scopes": []
  +  "scopes": [undefined]

Reproduces how often: [What percentage of the time does it reproduce?]

Every time.

Versions

• The output of mocha --version and node node_modules/.bin/mocha --version: 6.2.0, but the problem exists in v8.0.1 too
• The output of node --version: 12.8.x
• Your operating system
  • name and version: n/a
  • architecture (32 or 64-bit): n/a
• Your shell (e.g., bash, zsh, PowerShell, cmd): n/a
• Your browser and version (if running browser tests): n/a
• Any third-party Mocha-related modules (and their versions): n/a
• Any code transpiler (e.g., TypeScript, CoffeeScript, Babel) being used (and its version): n/a

Additional Information

This is due to Mocha's internal stringify function using square brackets to denote different types: https://github.com/mochajs/mocha/blob/v8.0.1/lib/utils.js#L303-L307

It looks okay when you're just doing a diff between undefined and something else, where the output will be:

- "something"
+ [undefined]

But because the stringify function is used recursively inside objects, it leads to confusing output.