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fix(testing): change assertThrows and assertThrowsAsync return type to void and Promise<void> #1052

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Jul 28, 2021
Merged

fix(testing): change assertThrows and assertThrowsAsync return type to void and Promise<void> #1052

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7dbfee3
docs: add note about `assertThrowsAsync` which needs to be `await`
lowlighter Jul 21, 2021
9ce3d1d
fix: change `assertThrowsAsync` return type to `Promise<void>`
lowlighter Jul 22, 2021
fe044a8
fix: update tests which were using `assertThrowsAsync` return value
lowlighter Jul 22, 2021
b9ea0eb
fix: change `assertThrows` return type to `Promise<void>`
lowlighter Jul 22, 2021
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4 changes: 4 additions & 0 deletions testing/README.md
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Expand Up @@ -108,6 +108,10 @@ Deno.test("fails", function (): void {

Using `assertThrowsAsync()`:

> ⚠️ Note that it is _required_ to `await` for `assertThrowsAsync` to ensure
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This technically isn't true. The promise returned from assertThrowsAsync can just be returned as well.

It would be more accurate to state that assertThrowsAsync returns a promise that will be resolved if the assertion passes, or rejected if it fails. This promise should be be awaited or be the return value from the test function.

Also, it would be far better to put it in the inline documentation for the API.

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This technically isn't true. The promise returned from assertThrowsAsync can just be returned as well.
This promise should be be awaited or be the return value from the test function.

This is actually not possible unless you void it. Trying to return the promise directly will make TypeScript complain because the test function expects void | Promise<void>:

error: TS2322 [ERROR]: Type 'Promise<Error>' is not assignable to type 'void | Promise<void>'.
  Type 'Promise<Error>' is not assignable to type 'Promise<void>'.
    Type 'Error' is not assignable to type 'void'.
Deno.test("2", () => assertThrowsAsync(async () => {}, Error));
                     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    at file:///deno_std/test.ts:6:22

    The expected type comes from the return type of this signature.
      export function test(name: string, fn: () => void | Promise<void>): void;
                                             ~~~~~~~~~~~~~~~~~~~~~~~~~~
        at asset:///lib.deno.ns.d.ts:188:42

It would be more accurate to state that assertThrowsAsync returns a promise that will be resolved if the assertion passes, or rejected if it fails. This promise should be be awaited or be the return value from the test function.

I'll change the wording to reflect the first part of your suggestion and update the inline documentation at the top.
The last part about being the return value from the test function is not valid though (at least currently)

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assertThrowsAsync shouldn't resolve with an error. I didn't realise that. Assertions shouldn't have return values.

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@lowlighter lowlighter Jul 22, 2021 •
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Seems that both assertThrows and assertThrowsAsync actually had Error and Promise<Error> instead of void and Promise<void> as return type so I patched them.

Assertions shouldn't have return values.

I agree with that statement, but it does feel awkward to retrieve error class from within the test function now.
At least it makes them consistent with other assertions return type and make it possible to avoid brackets use from inline arrow functions

> that all promises get resolved within the test `Deno.test` callback. Failing
> to do so may result in early exit or triggering the Op sanitizer

```ts
Deno.test("doesThrow", async function () {
await assertThrowsAsync(
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