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Learn about the various timeout options available in the fast-check.

How and where to stop?

When dealing with timeouts in property-based testing, there are several levels and options to consider. Timeouts can be applied to the entire test suite, limiting the total execution time of all tests. Alternatively, timeouts can be set for individual predicate executions, allowing for finer-grained control over the test time. Additionally, timeouts can be used to prevent excessively long test runs or to report on runs that have taken too long.

Let's dig into the multiple timeout options provided by fast-check.

At predicate level


You can use the timeout option with the assert function in fast-check to limit the amount of time allocated to run each instance of the predicate defined by your property. If the predicate takes longer than the specified time, the execution will be reported as a failure. fast-check will then attempt to shrink the inputs so that you can more easily identify the cause of the timeout.

Need asynchronous properties

It's important to note that the timeout option only works with asynchronous properties as it needs a way to interrupt another running script. If you want to use it with synchronous code, you can check out the @fast-check/worker package.

Let's explore how the timeout option works by looking at the following code snippet:

await fc.assert(
fc.asyncProperty(packagesArb, fc.nat(), async (packages, selectedSeed) => {
// Arrange
const allPackagesNames = Array.from(packages.keys());
const selectedPackage = allPackagesNames[allPackagesNames.length % selectedSeed];

// Act
const registry = new FakeRegistry(packages);
const dependencies = await extractAllDependenciesFor(selectedPackage, registry);

// Assert
for (const dependency of dependencies) {
{ timeout: 1000 },

In the provided example, the timeout will only be triggered if one execution of async (packages, selectedSeed) => {...} takes more than 1 second. It's also important to highlight the fact that the timeout option can only intervene for asynchronous tasks taking too long. In other words, in the predicate above, only the code executed asynchronously during the execution of extractAllDependenciesFor could be bypassed and raise a timeout issue.

Cannot stop the async code

It's important to note that fast-check cannot stop the execution of a running Promise as there is no way to cancel it in JavaScript. As a result, if a run takes too long to execute and exceeds the specified timeout limit, fast-check will simply ignore the follow-up results. This means that the code will continue to run until it completes, even if fast-check reported a timeout failure.

If you want to stop asynchronous code abruptly when it takes too long, you can check out the @fast-check/worker package. It provides a way to run code in a separate worker thread and stop the worker thread if it takes too long, effectively interrupting the execution of the code.

In case of failure linked to a timeout, the report might look like:

Uncaught Error: Property failed after 1 tests
{ seed: 1234070620, path: "0:0", endOnFailure: true }
Counterexample: [new Map([["my-package",{}]]),0]
Shrunk 1 time(s)
Got Property timeout: exceeded limit of 1000 milliseconds

Hint: Enable verbose mode in order to have the list of all failing values encountered during the run
at buildError (/workspaces/fast-check/packages/fast-check/lib/check/runner/utils/RunDetailsFormatter.js:131:15)
at asyncThrowIfFailed (/workspaces/fast-check/packages/fast-check/lib/check/runner/utils/RunDetailsFormatter.js:148:11)
at runNextTicks (node:internal/process/task_queues:60:5)
at process.processTimers (node:internal/timers:509:9)
Interaction with beforeEach and afterEach

Note that the function provided to beforeEach and afterEach are not included in the measured time for the timeout. If the execution is interrupted due to a timeout, afterEach will be called immediately without waiting for the predicate to finish.

Resources: API reference.
Available since 0.0.11.

At runner level


The interruptAfterTimeLimit option can be used to customize the maximum amount of time that the runner is allowed to execute a property. It works on both synchronous and asynchronous properties.

By default, interrupting a runner after the deadline is not considered an error unless no predicate succeeded. However, this behavior can be overridden by setting markInterruptAsFailure: true in which case any interruption of the execution will be considered a failure.

Here is a summary:

Interrupted...Resulting status with markInterruptAsFailure: falseResulting status with markInterruptAsFailure: true
without any successFailureFailure
with at least one successSuccessFailure
during shrink phaseFailure (shrink only happens on failures)Failure
Companion for Fuzzing

interruptAfterTimeLimit is particularly useful for fuzzing. For instance, setting it to interruptAfterTimeLimit: 600_000 and adding numRuns: Number.POSITIVE_INFINITY would allow the runner to loop for 10 minutes, regardless of the number of predicates executed during that time.

Resources: API reference.
Available since 1.19.0.


Interrupting the execution of predicates is one way to handle deadlines, but another option is skipping. skipAllAfterTimeLimit allows skipping the execution of predicates after the deadline has been reached.

Skipping predicates while there were no reported failures will result in a failure:

Failed to run property, too many pre-condition failures encountered
{ seed: 1119647454 }

Ran 0 time(s)
Skipped 10001 time(s)

Hint (1): Try to reduce the number of rejected values by combining map, flatMap and built-in arbitraries
Hint (2): Increase failure tolerance by setting maxSkipsPerRun to an higher value
Hint (3): Enable verbose mode at level VeryVerbose in order to check all generated values and their associated status
at buildError (/workspaces/fast-check/packages/fast-check/lib/check/runner/utils/RunDetailsFormatter.js:131:15)
at asyncThrowIfFailed (/workspaces/fast-check/packages/fast-check/lib/check/runner/utils/RunDetailsFormatter.js:148:11)
at process.processTicksAndRejections (node:internal/process/task_queues:95:5)

During the shrinking process, skipping predicates will result in one-by-one skipping of all the executions required by the shrinker.

Interrupting is more efficient

When we skip a predicate due to the skipAllAfterTimeLimit option, we still pass on it, which may take time. This is because each subsequent run needs to be marked as "will not be executed" one by one. On the other hand, with the interruptAfterTimeLimit option, the runner is stopped immediately when the deadline is reached, resulting in a faster stop.

Resources: API reference.
Available since 1.15.0.

All timeout options

OptionLevelProperty kindbeforeEach/afterEach included in the measured timeMark run as failed
interruptAfterTimeLimitrunnersync and asyncyesno except when first run or markInterruptAsFailure:true
skipAllAfterTimeLimitrunnersync and asyncyesno except when timeout occured outside of the shrink phase
Always run beforeEach and afterEach

beforeEach and afterEach functions will always be executed, regardless of whether they are included in the measured time for the timeout or not