types 3.0: add type tests

[filmaj]

Using a package like tsd, we can write basic tests for the type interfaces. For more complex types this can be extremely useful, especially during refactoring, to ensure the end-user experience stays consistent / we don't regress on the developer experience that TS provides.

[mrazauskas]

Consider employing TSTyche for this job, https://github.com/tstyche/tstyche

It can test using different versions of TypeScript (tstyche --target 4.8,5.2); is lightweight (~225kB); has --watch mode; and much more (for instance, documentation website: https://tstyche.org/guide/introduction).

[filmaj]

Thanks for the suggestion @mrazauskas. We have been using tsd in other projects of ours, like bolt-js (you can see its type tests here).

[mrazauskas]

Sure. It is up to you. I was chiming in, because you wrote “a package like tsd”.

Simply find five minutes and give TSTyche a spin. I am sure you will like it (;

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Indeed, tsd is very popular library. But let me show you something:

import { expectAssignable, expectNotAssignable, expectNotType, expectType } from "tsd";

type T = { a: 1 };

expectType<T>({ a: 1 }); // pass!
expectNotType<T>({ a: 1 }); // pass! hm.. what?!

expectAssignable<T>({ a: 1 }); // pass!
expectNotAssignable<T>({ a: 1 }); // pass! wait a moment..

That’s a bug which known for more than two years (tsdjs/tsd#141). And it is easy to fix. The problem is that positive assertions and their negated counterparts are evaluated using different logic instead of !result. I was maintaining a forked tsd-lite, it did not have this issue. Also many limitation were fixed (for instance, here is one you know already: slackapi/bolt-js#1053 (comment))

That was fun project, but I wanted to have goodies like test.skip(), test.only() or expect.fail(). Of course, one can use comments like // fix me: or so. That works, but I wanted a better type testing tool. That’s how TSTyche was born.

As I mentioned, its install size is only ~225kB. Zero dependencies. Works like a clock, packed with goodies, tests on specific version of TypeScript, has --watch mode, etc. That is way more than tsd can do, but tsds install size is 48.0MB. Obviously it comes with TypeScript baked in, but you already have it installed. Or? And also install size of the latest typescript is only 21.4MB. Oh well..

Don’t get me wrong. I think tsd is a useful project. But, as it happens with open source, have to admit that it got abandoned (here is an illustration: tsdjs/tsd#196).

[filmaj]

Great points; I will certainly take a look at TSTyche, thank you!