toContain gives unhelpful error message when value being checked is an object

[philraj]

🐛 Bug Report

expect(arr).toContain(val) gives an unhelpful error message when matching object values.

To Reproduce

Steps to reproduce the behavior:

1. Create an array containing objects: const arr = [{ a: 1 }, { b: 2}];
2. Try to expect(arr).toContain({ a: 1 })

Error message is displayed:

Expected array:
      [{"a": 1 }, {"b": 2}]
    To contain value:
      {"a": 1}

but mentions nothing about how identity is being used for equality checks, leaving you scratching your head since it plainly displays "the same" object literal in both places.

Expected behavior

It would ideally add a note about how it's checking object identity and not key/value equality.

Link to repl or repo (highly encouraged)

https://repl.it/repls/UsableLovingPhases

Run npx envinfo --preset jest

  System:
    OS: macOS High Sierra 10.13.6
    CPU: x64 Intel(R) Core(TM) i7-6820HQ CPU @ 2.70GHz
  Binaries:
    Node: 9.0.0 - /usr/local/bin/node
    Yarn: 1.7.0 - /usr/local/bin/yarn
    npm: 6.1.0 - /usr/local/bin/npm
  npmPackages:
    @types/jest: ^21.1.8 => 21.1.10
    jest: ^21.2.1 => 21.2.1

[Gregjarvez]

Expected array:
      [{"a": 1}, {"b": 2}]
 To contain value by reference:
     {"a": 1}

if this wording is okay I will make a PR for this.

[thymikee]

Oh, I didn't see you're using v21. In v23 we've added this message:

However, you can see it's a bit off. @Gregjarvez feel free to add 2 extra newlines here: https://github.com/facebook/jest/blob/277c547edd13ef2b69049ec1b4e768a2333471e1/packages/expect/src/matchers.js#L316

[Gregjarvez]

sure !!

[philraj]

Oh my mistake, was working on an existing project and didn't realize we weren't up to date. Thanks. :-)

[rahilvora]

can I work on this issue, if someone is not ?

[thymikee]

Too late! Already addressed:

[github-actions]

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