Functional written code is usually easier to test with a unit test. A unit test is an automated test that checks whether a small piece of code is working. We will use the ava test runner, which we can install using NPM:
npm install ava --save-dev npm install ava -g
At the time of writing, ava does not have type definitions bundled. We will quickly add some types for it in lib/test/ava.d.ts:
declare module "ava" {
function test(name: string, run: (t: any) => void): void;
namespace test {}
export = test;
}
We can now write some tests for the factorial function. In lib/test/utils.ts, we write a test case. Any test file will have the following structure:
import * as test from "ava";
import { factorial } from "../model/utils";
test("factorial", t => {
t.is(factorial(0), 1);
t.is(factorial(1), 1);
t.is(factorial(2), 2);
t.is(factorial(3), 6);
t.is(factorial(4), 24);
}); ...