Add option for cleaner error reporting in fc.assert

[alex1yaremchuk]

🚀 Feature Request

When using fc.assert in modern test runners like Vitest or Jest, the error output tends to be very verbose — with stack traces originating deep from inside fast-check internals.

We'd love a cleaner, user-focused error like:

❌ Property failed after 3 tests. Failing inputs: [0, 100001]
Ideally, this could be solved by:

Allowing a custom errorFormatter

Or a suppressStackTrace option

Or a lightweight fc.assertClean alternative

This would help a lot with DX and improve the readability of PBT failures.

Motivation

Cleaner tests output without stack traces

Example

Current output is way too dirty -

 FAIL  tests/add.test.js > add(a, b) > должен быть коммутативен
Error: Property failed after 1 tests
{ seed: 2045225418, path: "0:0:1:0:6:0:1:3:0:0:0", endOnFailure: true }
Counterexample: [0,100001]
Shrunk 10 time(s)

Hint: Enable verbose mode in order to have the list of all failing values encountered during the run
 ❯ buildError node_modules/fast-check/lib/check/runner/utils/RunDetailsFormatter.js:156:19
 ❯ throwIfFailed node_modules/fast-check/lib/check/runner/utils/RunDetailsFormatter.js:165:11
 ❯ reportRunDetails node_modules/fast-check/lib/check/runner/utils/RunDetailsFormatter.js:178:16
 ❯ Module.assert node_modules/fast-check/lib/check/runner/Runner.js:61:9
 ❯ tests/add.test.js:31:8
     29| describe('add(a, b)', () => {
     30|   it('должен быть коммутативен', () => {
     31|     fc.assert(
       |        ^
     32|       fc.property(
     33|         fc.integer({min: -1e6, max: 1e6 }),

Caused by: Error: Property failed by returning false
 ❯ Property.run node_modules/fast-check/lib/check/property/Property.generic.js:46:28   
 ❯ runIt node_modules/fast-check/lib/check/runner/Runner.js:14:30
 ❯ check node_modules/fast-check/lib/check/runner/Runner.js:54:11
 ❯ Module.assert node_modules/fast-check/lib/check/runner/Runner.js:57:17
 ❯ tests/add.test.js:31:8

[dubzzz]

Thanks for having reported this issue. If I'm not wrong this is the exact purpose of the parameters called reporter and asyncReporter that you can provide to the assert function provided by fast-check. If you want your reporter to be used everywhere (ie for all calls to assert) you can configure it globally by passing it to configureGlobal.

More details: here.

Hope it solves your issue 👍

[alex1yaremchuk]

Thank you very much for kindly pointing that out.

Works like a charm!

[alex1yaremchuk]

Hi Nicolas!

I met another obstacle with this attitude.

With custom reporter I get clean output locally, but
I don't get exit code 1 for failed tests in CI.

Isn't there a way to have both - exit code 1 and clean output?

do I have to analyze output for keywords to know test failed, or there is a better way?

Sorry for disturbance with another probably silly question!

Thank you!

Thanks for having reported this issue. If I'm not wrong this is the exact purpose of the parameters called reporter and asyncReporter that you can provide to the assert function provided by fast-check. If you want your reporter to be used everywhere (ie for all calls to assert) you can configure it globally by passing it to configureGlobal.

More details: here.

Hope it solves your issue 👍

[dubzzz]

You normally have to throw in case it's an error case as we do within our default reporters:

/** @internal */
function throwIfFailed<Ts>(out: RunDetails<Ts>): void {
  if (!out.failed) return;
  throw buildError<Ts>(defaultReportMessage(out), out);
}

/** @internal */
async function asyncThrowIfFailed<Ts>(out: RunDetails<Ts>): Promise<void> {
  if (!out.failed) return;
  throw buildError<Ts>(await asyncDefaultReportMessage(out), out);
}

This is the default code for reporters.

[alex1yaremchuk]

Thank you!

I finished with clearing stack before throwing.