[!IMPORTANT] We're excited you're checking out Hegel! Hegel is in beta, and we'd love for you to try it and report any feedback.

As part of our beta, we may make breaking changes if it makes Hegel a better property-based testing library. If that instability bothers you, please check back in a few months for a stable release!

See https://hegel.dev/compatibility for more details.

Hegel for TypeScript

• Documentation
• Website

Hegel is a property-based testing library for TypeScript. Hegel is based on Hypothesis, using the Hegel protocol.

Installation

To install: npm install --save-dev @hegeldev/hegel.

Hegel requires Node 16+. Bun and Deno are not currently supported.

Hegel will use uv to install the required hegel-core server component. If uv is already on your path, it will use that, otherwise it will download a private copy of it to ~/.cache/hegel and not put it on your path. See https://hegel.dev/reference/installation for details.

Quickstart

Here's a quick example of how to write a Hegel test:

import { test } from "vitest";
import * as hegel from "@hegeldev/hegel";
import * as gs from "@hegeldev/hegel/generators";

function mySort(ls: number[]): number[] {
  const result = [...ls].sort((a, b) => a - b);
  return [...new Set(result)];
}

test("my_sort matches builtin", () =>
  hegel.test((tc) => {
    const vec1 = tc.draw(gs.arrays(gs.integers()));
    const vec2 = mySort(vec1);
    const sorted = [...vec1].sort((a, b) => a - b);
    if (JSON.stringify(sorted) !== JSON.stringify(vec2)) {
      throw new Error(`sort mismatch: ${JSON.stringify(sorted)} != ${JSON.stringify(vec2)}`);
    }
  }));

This test will fail when run with vitest! Hegel will produce a minimal failing test case for us:

Draw 1: [0, 0]
Error: sort mismatch: [0,0] != [0]

Hegel reports the minimal example showing that our sort is incorrectly dropping duplicates. If we remove the new Set(...) deduplication from mySort(), this test will then pass (because it's just comparing the standard sort against itself).

Async tests

For async tests, use hegel.testAsync:

import { test } from "vitest";
import * as hegel from "@hegeldev/hegel";
import * as gs from "@hegeldev/hegel/generators";

test("fetch returns a value matching its input", () =>
  hegel.testAsync(async (tc) => {
    const id = tc.draw(gs.integers({ minValue: 1, maxValue: 1000 }));
    const result = await fetchUser(id);
    if (result.id !== id) {
      throw new Error(`Expected id=${id}, got ${result.id}`);
    }
  }));