Values

Value plugins customize which values get forwarded to your predicates. Bias plugins customize how generation targets values, while equal-values plugins avoid executing your predicates twice on the same value.

Equal-values plugins keep track of the values already covered during the run and replay the outcome of the first execution whenever a value gets generated again. Values are compared based on their stringified representation.

unbiased

The unbiased plugin generates the values feeding your predicates without any bias.

{
  plugins: [
    unbiased(), // draw all values from the full range of the arbitraries
  ];
}

Without this plugin, generation is biased by default. Compared with a uniform distribution, smaller or more extreme values are more likely to be generated, helping uncover common issues earlier. For example, with fc.integer(), small values such as 1 and 2 are more likely to be generated than 171414468.

With this plugin enabled, every run draws from the full range of the arbitraries with no special treatment. For fc.integer(), 171414468 is then just as likely to be generated as 1 or 2.

Resources: API reference.

ignoreEqualValues

The ignoreEqualValues plugin discards runs on already covered cases.

{
  plugins: [
    ignoreEqualValues(), // never execute the predicate twice on the same value
  ];
}

Discarded runs still count as runs: a run replaying a past success stays a success.

Resources: API reference.

skipEqualValues

The skipEqualValues plugin skips runs on already covered cases.

{
  plugins: [
    skipEqualValues(), // never execute the predicate twice on the same value
  ];
}

Contrary to ignoreEqualValues, a run replaying a past success gets marked as skipped instead of successful: passed the maximal number of allowed skips the run will be marked as failed. It gives stronger guarantees on the number of distinct values covered by the run at the price of potentially failing on arbitraries unable to produce enough distinct values.

Resources: API reference.