TestCheck.js

Common errors

• TypeError: (0 , testcheck__WEBPACK_IMPORTED_MODULE_0__.check) is not a function

  cause This typically occurs when mixing CommonJS `require` with ES Modules `import` syntax, or when a bundler incorrectly transpiles the module, or if `check` is attempted to be used as a default import when it's a named export.
  fix Ensure consistent import style: use `import { check, gen, property } from 'testcheck';` for ES Modules/TypeScript environments, or `const { check, gen, property } = require('testcheck');` for CommonJS environments.

• ReferenceError: check is not defined

  cause The `check` function (or `gen`, `property`) was not imported or required correctly into the current JavaScript scope before being used.
  fix Add the necessary import statement at the top of your file: `import { check, gen, property } from 'testcheck';` (for ESM/TS) or `const { check, gen, property } = require('testcheck');` (for CommonJS).

• Property function did not return true

  cause This is TestCheck.js reporting a test failure. The property function provided returned `false` (or threw an error) for one or more generated test cases. This indicates either a bug in the code under test or an incorrect/flawed property definition.
  fix Examine the `result.shrunk.smallest` output from the `check` function to identify the minimal failing input. Debug your property function and/or the application code it tests to resolve the discrepancy.

Warnings

• breaking The `1.0.0-rc.0` release introduced significant changes and new features, implying potential breaking API changes for users upgrading from pre-1.0 (e.g., 0.x) versions. A full report was promised for the final `1.0.0` release.
  Fix: Consult the official API documentation (http://leebyron.com/testcheck-js/api) and the Walkthrough Guide (http://leebyron.com/testcheck-js/guide) for updated API usage and migration details.
• gotcha TestCheck.js is a property testing *utility* and does not include a test runner. It must be integrated with an existing test framework (like AVA, Jasmine/Jest, Mocha, or Tape) or its `check` function called explicitly. It will not automatically discover and run tests.
  Fix: If using a test framework, install and configure the relevant `testcheck` integration package (e.g., `ava-check`, `jasmine-check`). Otherwise, invoke `check()` directly within your testing environment.
• gotcha As a release candidate (`1.0.0-rc.2`), the API is generally stable but may still undergo minor adjustments or refinements before the final `1.0.0` release. Users should be aware of potential, albeit small, changes.
  Fix: Monitor the official GitHub repository and documentation for updates leading up to the final `1.0.0` release. Ensure your test suite has adequate coverage to detect any unexpected behavior from minor API shifts.

Install

• npm install testcheck npm
• yarn add testcheck yarn
• pnpm add testcheck pnpm

Imports

• check
  wrong:

  const check = require('testcheck');

  correct:

  import { check } from 'testcheck';

  The primary function to execute property tests. For ES Modules and TypeScript, use a named import. Using `require` in an ESM context will likely fail.
• gen
  wrong:

  import * as gen from 'testcheck/gen';

  correct:

  import { gen } from 'testcheck';

  An object providing various predefined data generators (e.g., `gen.int`, `gen.string`, `gen.array`). It is a named export from the main package.
• property
  wrong:

  import Property from 'testcheck';

  correct:

  import { property } from 'testcheck';

  Used to encapsulate a predicate function into a testable property. It is available as a named export; do not attempt a default import.
• { check, gen, property } (CommonJS)
  wrong:

  import { check, gen, property } from 'testcheck';

  correct:

  const { check, gen, property } = require('testcheck');

  For CommonJS environments, destructure named exports directly from the `require` call. Mixing ES Modules import syntax with CommonJS `require` in the same file can lead to unexpected behavior.

Quickstart

This quickstart demonstrates how to define and run a basic property test using `gen.int` and `check`, illustrating both success and failure reporting.

import { check, gen, property } from 'testcheck';

// Define a property: any integer subtracted from itself should be 0.
// TestCheck will generate various integer inputs (x) and check this property.
const subtractionProperty = property(
  gen.int, // Generator for an integer input
  (x: number) => x - x === 0
);

// Run the property check with default settings
const result = check(subtractionProperty);

// Log the outcome. In a real test suite (e.g., Jest, Mocha), you'd use
// framework-specific assertions based on `result.pass`.
if (result.pass) {
  console.log('Property holds true for all generated cases after ' + result.numTests + ' tests.');
} else {
  console.error(`Property failed after ${result.numTests} tests.`);
  console.error(`Failing input (minimal): ${JSON.stringify(result.shrunk.smallest)}`);
  // Access original failing input: result.fail
}

// Example with multiple generators and a more complex property
const additionCommutativity = property(
  gen.int, gen.int,
  (a: number, b: number) => a + b === b + a
);

const addResult = check(additionCommutativity);
if (!addResult.pass) {
  console.error(`Addition commutativity failed for: ${JSON.stringify(addResult.shrunk.smallest)}`);
}