Everything You Need for Type Testing.

Helpers

If you are used to test JavaScript, a simple type test file should look familiar:

import { expect, test } from "tstyche";
 
function isSameLength<T extends { length: number }>(a: T, b: T) {
  return a.length === b.length;
}
 
test("isSameLength", () => {
  expect(isSameLength([1, 2], [1, 2, 3])).type.toBe<boolean>();
  expect(isSameLength("one", "two")).type.toBe<boolean>();
 
  expect(isSameLength).type.not.toBeCallableWith(1, 2);
});

To organize, debug and plan tests TSTyche has:

• test(), it() and describe() helpers,
• with .only, .skip and .todo run mode flags.

Assertions

The assertions can be used to write type tests (like in the above example) or mixed in your unit tests:

import assert from "node:assert";
import test from "node:test";
import * as tstyche from "tstyche";
 
function milliseconds(value: number) {
  if (typeof value === "number" && !Number.isNaN(value)) {
    return value * 1000;
  }
 
  throw new Error("Not a number");
}
 
test("milliseconds", () => {
  const sample = milliseconds(10);
 
  assert.equal(sample, 10_000);
  tstyche.expect(sample).type.toBe<number>();
 
  // Will pass as a type test and not throw at runtime
  tstyche.expect(milliseconds).type.not.toBeCallableWith("20");
});

Here is the list of all matchers:

• .toBe(), .toBeAssignableTo(), .toBeAssignableWith() compare types or types of expression,
• .toAcceptProps() checks the type of JSX component props,
• .toBeApplicable ensures that the decorator function can be applied,
• .toBeCallableWith() checks whether a function is callable with the given arguments,
• .toBeConstructableWith() checks whether a class is constructable with the given arguments,
• .toHaveProperty() looks up keys on an object type.

Runner

The tstyche command is the heart of TSTyche. For example, it can select test files by path, filter tests by name and pass them through a range of TypeScript versions:

tstyche query-params --only multiple --target '>=5.0 <5.3'

This simple! (And it has watch mode too.)