Codecov Report

Merging #9626 into master will increase coverage by 0.00%.
The diff coverage is 79.31%.

@@           Coverage Diff           @@
##           master    #9626   +/-   ##
=======================================
  Coverage   64.90%   64.90%           
=======================================
  Files         288      288           
  Lines       12195    12197    +2     
  Branches     3022     3022           
=======================================
+ Hits         7915     7917    +2     
  Misses       3639     3639           
  Partials      641      641           

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ci/circleci: test-browser:

error https://registry.yarnpkg.com/path-exists/-/path-exists-3.0.0.tgz: Extracting tar content of undefined failed, the file appears to be corrupt: "ENOSPC: no space left on device, write"

😬

Failing test seems to be unrelated - it's failing on a different worker related test on each action

Note: this breaks a couple of tests in @types/jest's test suite:

expect.addSnapshotSerializer({
    print: (value: {}) => '',
    test: (value: {}) => value === value,
});

expect.addSnapshotSerializer({
    print: (value: {}, serialize: (val: {}) => string, indent: (str: string) => string, opts: {}) => '',
    test: (value: {}) => value === value,
});

unknown is not assignable to {}, so the first value now has to have the type unknown instead:

expect.addSnapshotSerializer({
    print: (value: unknown) => '',
    test: (value: {}) => value === value,
});

expect.addSnapshotSerializer({
    print: (value: unknown, serialize: (val: {}) => string, indent: (str: string) => string, opts: {}) => '',
    test: (value: {}) => value === value,
});

No idea if this example is realistic or not, I just noticed it while running the TS team's Definitely Typed tests. It seems reasonable that a serialiser should handle any JS value, including null and undefined.

I think it makes sense. Ideally test would work as a typeguard for print, but I'm not sure how to achieve that?

I think it makes sense.

if it doesn't then the type should be object or narrower ;)

Ideally test would work as a typeguard for print

If I understand correctly:

type Serializer<T> = {
  print: (val: T, serialize: JestSerialize, indent: number) => string;
  test: (val: unknown) => val is T;
};

I can't find the actual places in code where this is used, but that should do the trick if you're just doing:

if(serializer.test(val)) {
  serializer.print(val, /* ... everything else ... */);
}

Oh also rereading @sandersn's example I think while yes the type is more accurate now, the value parameter should have the same type for both functions

i.e

// incorrect
expect.addSnapshotSerializer({
    print: (value: unknown) => '',
    test: (value: {}) => value === value,
});

// accurate
expect.addSnapshotSerializer({
    print: (value: unknown) => '',
    test: (value: unknown) => value === value,
});

// spot on
expect.addSnapshotSerializer({
    print: (value: MyType) => '',
    test: (value: unknown): value is MyType => value === value,
});

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