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Generative property testing for Tape

tape-check adds the generative testing power of testcheck-js to Tape. This allows some of your Tape tests to accept arguments and ensure your tests pass not just under your contrived test cases but also pass for hundreds of randomly generated test cases.

Getting started

Install tape-check using yarn.

yarn add --dev tape-check

Or using npm

npm install --save-dev tape-check

Then include tape-check in your test.

const { check, gen } = require('tape-check')


const test = require('tape')
const { check, gen } = require('tape-check')

test('addition is commutative', check(gen.int, gen.int, (t, numA, numB) => {
  t.equal(numA + numB, numB + numA)

The gen object is provided directly by testcheck and defines what type of values to generate. The test will be run numerous times with randomly generated values, ensuring all expectations are met for every run. If a test expectation fails, then the test will re-run with "smaller" values until the smallest failing value is found which can better help explain edge cases with your test and produce consistent results, despite being initially fueled by randomness.

For example, here's a test which we expect would fail:

const test = require('tape')
const { check, gen } = require('tape-check')

test('division is commutative', check(gen.sPosInt, gen.sPosInt, (t, numA, numB) => {
  t.equal(numA / numB, numB / numA)

When we run this test, we find the smallest failing test:

> tape test.js
TAP version 13
# division is commutative
not ok 1 should be equal ( 1, 2 )
    operator: equal
    expected: 0.5
    actual:   2
    at: Test.check (test.js:6:5)

# tests 1
# pass  0
# fail  1


If a test is taking a long time, needs to generate larger values, or should be run with a consistent random seed, you can alter the behavior with options:

  times: number;   // the number of test cases to run. Default: 100
  maxSize: number; // the maximum "size" of the test data. Default: 200
  seed: number;    // defaults to a random value from 1 to 2^32-1.

To use these options with your check, include an options object before the argument generators.

test('runs 10 times', check({ times: 10 }, gen.posInt, (t, n) => {
  t.true(x >= 0)

To learn more about property testing, or to learn about the tapeilable value generators, check out testcheck.