Opt-out special char escaping in result diff

[maciej-ka]

What

At the moment jest will always escape special char \ in diff output of the failed test and replace it with \\. I would love to have an option to disable this.

Having a test like this:

it('works', () => {
  const str = 'a\\nb';
  expect(str).toEqual('');
});

(note that console.log(str) will output a\nb)

Message is:

    expect(received).toEqual(expected)

    Expected value to equal:
      ""
    Received:
      "a\\nb"

And the desired output would be:

    expect(received).toEqual(expected)

    Expected value to equal:
      ""
    Received:
      "a\nb"

Why

In my test suite, diff will print a valid javascript.

It's about a library to write custom parsers. Example test is like this:

  test('basic match', () => {
    const syntax = many(char('a'));
    const actual = syntax(new Cursor('aa'));
    expect(pretty(actual)).toEqual(pretty(
      manyNode(0, 2,
        charNode(0, 1, 'a'),
        charNode(1, 2, 'a'))
    ));
  });

Function pretty takes a tree of js objects and returns a string that is a valid javascript.

When expected tree structure is complicated, it's very handy to start with empty expectation:

    expect(pretty(actual)).toEqual(pretty(
    ));

And copy-paste one from test failure message:

    Expected value to equal:
      undefined
    Received:
      "
    manyNode(0, 2,
      charNode(0, 1, 'a'),
      charNode(1, 2, 'a'))
    "

However, at the moment, there is no way jest can print \n in that message, because it will always escape it into \\n.

Posible solution

Make escaping of a special chars optional in pretty-format and introduce an option in jest that would be passed to that line of code.

[SimenB]

Ok, landed the option to pretty-format. Now, we need to figure out some way to allow us to pass that option down to it (via jest-diff, I suppose). @rickhanlonii @thymikee @pedrottimark ideas?

[rickhanlonii]

IMO this should a custom matcher, with that you can pass the option directly to jest-diff

[github-actions]

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