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Core Blocks

Every fast-check test is built from the same three pieces:

  1. Arbitraries describe what values to generate. They pair a random generator with a shrinker so that failing inputs collapse back to readable counterexamples.
  2. Properties describe what must hold for those values. A property takes one or more arbitraries and a predicate. It asserts that for any input the predicate stays valid.
  3. Runners describe how to execute the property: how many runs, with what seed, how to report failures, whether to throw or return details.

The children below are the reference pages for each block: start with Properties if you have never written one, jump to Arbitraries when you need the right generator and come back to Runners when you need to tune execution.

:::tip Reference, not tutorial The Core Blocks pages are the exhaustive reference. If you are looking for a guided, hands-on walkthrough instead, start with the Quick Start tutorial. :::

Last updated on by Nicolas DUBIEN