This started out as a crazy idea I had during a Stainless hackathon! Let's see what y'all do with it 👀
11.02.2026 20:46
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This started out as a crazy idea I had during a Stainless hackathon! Let's see what y'all do with it 👀
have been checking out lfi recently - fairly small library of very useful async utils
in repos that pull a lot of separate async util packages in, this could be a good unified replacement
Lovely to hear you're finding it useful :)
And thanks for sponsoring! Definitely let me know if you run into any issues
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My solution for today! Pretty happy with how this one turned out
gist.github.com/TomerAberbac...
Well, I'll try to test more targeted slices next time to make sure I'm most likely not duplicating the code under test
Ya know, I think it's a bit more difficult for me to write a good property-based test when I don't see the implementation...
In this case I probably basically reimplemented the function under test when normally I would be more cognizant of that and would test a slice of the input space like you did
![const letterArb = fc.record({ id: fc.string() });
test("helping Santa", () => {
fc.assert(
fc.property(fc.array(letterArb), (letters) => {
const deduplicatedLetters = dropLettersFromDuplicatedSenders(letters);
const lettersById = Map.groupBy(letters, ({ id }) => id);
const expectedLetters = Array.from(
lettersById.values(),
(letters) => letters[0]
);
// Manually check length and individual items for reference equality.
expect(deduplicatedLetters.length).toBe(expectedLetters.length);
for (let i = 0; i < deduplicatedLetters.length; i++) {
expect(deduplicatedLetters[i]).toBe(expectedLetters[i]);
}
})
);
});](https://cdn.bsky.app/img/feed_thumbnail/plain/did:plc:dtrh3qqbjlu3ewyxo7tge3sa/bafkreihvznp4glyxulwuo4tqvakywnsuy76vl3wh6inmpj42kztha2e2ju)
const letterArb = fc.record({ id: fc.string() }); test("helping Santa", () => { fc.assert( fc.property(fc.array(letterArb), (letters) => { const deduplicatedLetters = dropLettersFromDuplicatedSenders(letters); const lettersById = Map.groupBy(letters, ({ id }) => id); const expectedLetters = Array.from( lettersById.values(), (letters) => letters[0] ); // Manually check length and individual items for reference equality. expect(deduplicatedLetters.length).toBe(expectedLetters.length); for (let i = 0; i < deduplicatedLetters.length; i++) { expect(deduplicatedLetters[i]).toBe(expectedLetters[i]); } }) ); });
My solution for today! Got to use some brand new ECMAScript features for it
![const nameArb = fc.stringMatching(/^[a-z]+$/);
const ageArb = fc.integer({ min: 7, max: 77 });
test("helping Santa", () => {
fc.assert(
fc.property(
fc
.tuple(ageArb, fc.array(nameArb))
.map(([age, names]) => [age, names.map((name) => ({ name, age }))]),
([age, letters]) => {
const sortedLetters = sortLetters(letters);
expect(sortedLetters.length).toBe(letters.length);
for (const letter of letters) {
expect(letter.age).toBe(age);
}
for (let i = 1; i < sortedLetters.length; i++) {
const previousLetter = sortedLetters[i - 1];
const currentLetter = sortedLetters[i];
// Failed here!
expect(previousLetter.name <= currentLetter.name).toBe(true);
}
}
)
);
});](https://cdn.bsky.app/img/feed_thumbnail/plain/did:plc:dtrh3qqbjlu3ewyxo7tge3sa/bafkreihyq2ctsqkdlbif4hpfx36nvvcpby2qqri3m775oqjw5gelzp5eny)
const nameArb = fc.stringMatching(/^[a-z]+$/); const ageArb = fc.integer({ min: 7, max: 77 }); test("helping Santa", () => { fc.assert( fc.property( fc .tuple(ageArb, fc.array(nameArb)) .map(([age, names]) => [age, names.map((name) => ({ name, age }))]), ([age, letters]) => { const sortedLetters = sortLetters(letters); expect(sortedLetters.length).toBe(letters.length); for (const letter of letters) { expect(letter.age).toBe(age); } for (let i = 1; i < sortedLetters.length; i++) { const previousLetter = sortedLetters[i - 1]; const currentLetter = sortedLetters[i]; // Failed here! expect(previousLetter.name <= currentLetter.name).toBe(true); } } ) ); });
Just went ahead and tried it! This worked right away
So close to being a teapot! (iykyk)
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Nice :)
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I encountered a strange bug while working on Docs at Google. Read about it on my blog!
tomeraberba.ch/when-two-bug...
So... Is this the place where the blue bird went?