[Feature]: Extend CustomMatcher instead of just functions

[jhwang98]

🚀 Feature Proposal

My goal is to create a better Buffer matcher. One that will show the diff by bytes and also diff by the encoded string.

    expect(received).bufferToBeEqualToBuffer(string)

    As buffers
    - Expected
    + Received

    @@ -4,11 +4,11 @@
          115,
          116,
          111,
          114,
          101,
    -     68,
    +     100,
          32,
          115,
          116,
          114,
          105,

    As strings
    - Expected
    + Received

    - +storeD string
    + +stored string

Unfortunately when I use it as an AsymmetricMatcher the format is generic.

Expected: bufferToBeEqualToBuffer<key-name>
Received: {"data": [43, 115, 116, 111, 114, 101, 100, 32, 115, 116, …], "type": "Buffer"}

I investigated but it doesn't seem like I have control of this string. https://github.com/facebook/jest/blob/main/packages/expect/src/jestMatchersObject.ts#L105

I'm not sure if this is the best approach but allowing developers to specify their own AsymmetricMatcher subclass would give us more control.

Motivation

This will allow jest-extended to have nicer messages for their AsymmetricMatchers.

Of course my use case will be improved as well.

Example

No response

Pitch

It doesn't seem possible to get this functionality by expect.extend or other avenues.

[jhwang98]

Loosely referenced in #4711 (comment)

[mrazauskas]

I think this is good idea. It should be possible to make expect.extend(matchers) to handle instances of AsymmetricMatcher. The type of its argument could be: Record<string, RawMatcherFn | AsymmetricMatcher>.

Another approach could be adding something like expect.addAsymmetricMatchers(asymmetricMatchers). Not sure if that is good to have two separate methods which do almost the same.

Would you be interested to open a PR to see how actual implementation could look like?

[jhwang98]

Yeah why not. My thoughts were to change expect.extend to accept AsymmetricMatchers.

Won't have too much time right now but I'll eventually get around to this.

(Anyone watching at home feel free to take this on)

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