For example, instead of testing a sorting function with just [3, 1, 2] , a property-based test would state a property like "for all arrays, the output should be sorted." The framework then generates hundreds or thousands of random arrays to test this property, a task that would be impossible to do manually.
Guilt? That was new. v 0.38 didn’t measure guilt.
import fc from 'fast-check'; // The function we want to test const reverseString = (str: string): string => [...str].reverse().join(''); // The property-based test test('Reversing a string twice always returns the original string', () => fc.assert( fc.property(fc.string(), (str) => return reverseString(reverseString(str)) === str; ) ); ); Use code with caution. Key Features That Defined the v0.39 Milestone
She swiped it away. Again.
We just pushed the latest draft/version of Fast Check! It is focused squarely on stability and performance. Quick highlights: Snappier performance across the board. Fixed a couple of annoying minor bugs. Kept the package lightweight and efficient. fast check v 0.39
catch (error) // Handle division by zero if (op === '/' && b === 0) return true;
If fast-check finds an input that breaks your property, it executes a process called . It automatically minimizes the failing input to the smallest, simplest counterexample possible, making debugging incredibly straightforward. 2. Core Concepts in v0.39
: If v0.39 finds a bug with a massive 1,000-character string, it doesn't just stop there. It "shrinks" that input down to the smallest possible value that still fails, making debugging significantly easier.
Fast-check v0.39 was a critical release in the history of property-based testing for the JavaScript and TypeScript ecosystems. It built upon the core principles introduced in earlier 0.x versions, adding features like web browser bundles, more flexible string generators, and important backward compatibility features. By understanding the concepts and methods introduced in this version, you can write tests that are not only more thorough but also smarter. Property-based testing with fast-check shifts the way you think about testing, moving from verifying specific examples to proving universal truths about your code, ultimately leading to more robust and reliable software applications. For example, instead of testing a sorting function
is a significant update for the widely used property-based testing (PBT) framework for JavaScript and TypeScript. This release continues the library’s mission of shifting testing from manual example-picking to automated, generative discovery of edge cases. Overview of Fast Check
Do not completely replace your traditional unit tests. Use example tests for happy paths and document behavior, and use fast-check to probe boundaries and edge cases. Set Limits on Complex Arbitraries
: Written in TypeScript, it offers strong type safety and excellent IDE integration. Key Features of v0.39
You can easily adjust the behavior of your runners by passing a configuration object to fc.assert : javascript Unlike example-based testing (e.g.
Fast-check is a property-based testing framework designed for JavaScript, TypeScript, and any other compile-to-JS languages. Unlike example-based testing (e.g., expect(add(1, 2)).toBe(3) ), which tests specific scenarios, property-based testing defines general properties that should hold true for any input.
fc.assert(property, numRuns: 1000, // Increase runs for deeper security seed: 42, // Replay a specific test run timeout: 2000, // Max time allowed for a single run in ms endOnFailure: true // Stop immediately when a failure occurs ); Use code with caution. 5. Best Practices for Property-Based Testing
Fast Check (often stylized as fast-check ) allows developers to define —statements about how a piece of code should behave for any valid input—rather than writing individual test cases.