Feature: toMatchSnapshot should let the caller know if the comparison was a match

[jasonnet]

Feature

Current behavior

Right now toMatchSnapshot calls dontThrow. This prevents the call to toMatchSnapshot from throwing an exception even if the match fails. This precludes the caller from detecting if the snapshot match failed.

Requested behavior

Provide a way for a test to determine if the toMatchSnapshot comparison failed. This could be

• an additional parameter to toMatchSnapshot. Or
• having toMatchSnapshot set a variable that could be checked subsequently by the test. Or
• some way for the caller to override the dontThrow method. Or
• toMatchSnapshot returning a value indicating the comparison results. Or
• an alternative to toMatchSnapshot that throws exceptions. Or.
• a expect().doThrow(). prefix that lets toMatchSnapshot know that it should not call dontThrow
• simply making the default toMatchSnapshot failure behavior to throw an exception like the other similar methods.

Benefits

• Provides an easy way for callers to detect failure and alter the direction of a test.
• Provides a way to halt the test immediately, allowing an engineer to inspect the state of the system immediately after the first sign of failure.
• Provides consistency with the rest of the expect() methods
• Provides better future-proofing than having the test author provide his own, 98% identical, implementation of toMatchSnapshot

[thymikee]

We already have .toThrowErrorMatchingSnapshot(), please see the docs: https://jestjs.io/docs/en/expect#tothrowerrormatchingsnapshot

[jeysal]

@thymikee this issue is actually about the expect(x).toMatchSnapshot() call itself throwing, not snapshotting the error thrown by a user-provided function.
@jasonnet if you're still interested in this, perhaps a concrete example would help understand why this could be useful in a test case.

[jasonnet]

@jeysal Thanks. You are correct that .toThrowErrorMatchingSnapshot doesn't appear to apply here because it seems to catch an exception, but the problem is that no exception is thrown.

It's been a while since I looked at this. This weekend I'll reboot my brain on jest and see if I can clarify further for @thymikee.

[thymikee]

Thanks!

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