Asserting that TypeScript should fail to type check some example code

Question

When I write a TypeScript library with generic constraints, I'd like to be able to specify tests of code that should type check and code that should not (because, say, I want to make sure the type checker is catching things that wouldn't satisfy a constraint correctly).

I haven't found a solution in conventional unit testing libraries (since the tests for failures wouldn't compile in the first place). I think I saw some examples of this deep in the TypeScript test suite where they either used file naming or comments to indicate assertions, but I can't find them now, and I couldn't unravel how the test runner worked for such a large project anyway. Googling is hard too: every combination of terms I've thought about brings back links about things like type guards or bug reports about TypeScript itself.

I know I could rig up a bash script that just runs tsc over the files and asserts that they fail, but I'd prefer the ability to match on specific compilation failures if possible. Any ideas?

jmattheis · Accepted Answer · 2018-07-05 16:57:12Z

7

You can use Microsoft/dtslint for this. After you went through the setup you can write test files like this:

index.d.ts:

// TypeScript Version: 2.1

declare module 'Abc' {
    function TestFunc(s: string): undefined;
}

test.ts:

import Abc from 'Abc';

// $ExpectError
Abc.TestFunc(true);

// $ExpectError
Abc.TestFunc(5);

Abc.TestFunc("it is a string");

Now when you run the dtslint cli tool no error will be returned, because all errors are expected. If f.ex. an error is not commented with $ExpectError (e.g. Abc.TestFunc(true);) then the dtslint tool fails with a message:

Error: C:/stackoveflow/test.ts:3:14
ERROR: 3:14  expect  TypeScript@next compile error:
Argument of type 'true' is not assignable to parameter of type 'string'.

    at I:\..\dtslint\bin\index.js:101:19
    at Generator.next (<anonymous>)
    at fulfilled (I:\..\dtslint\bin\index.js:5:58)
    at <anonymous>

answered Jul 5, 2018 at 16:57

jmattheis

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1 Comment

AndrewO Over a year ago

Hmm... seems to come with a lot of assumptions about my project setup that are more valid for a DefinitelyTyped contribution than a free-standing library. E.g.: tsconfig has certain keys, there's an index.d.ts, tslint extends dtslint's configuration. Maybe there are parts that I can use if I break about the standard behavior in the bin file?

michael · Accepted Answer · 2021-02-04 16:48:54Z

4

For tsc, use @ts-expect-error.

Snippet from @jmattheis example above:

// @ts-expect-error
Abc.TestFunc(true);

answered Feb 4, 2021 at 16:48

michael

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