Testing TypeScript

Jest (with ts-jest or Babel) runs TypeScript tests directly — the tests themselves read almost identically to plain JavaScript Jest tests, with the added benefit that a wrongly-typed test itself won't compile.

// calculator.ts
export function add(a: number, b: number): number {
    return a + b;
}

// calculator.test.ts
import { add } from "./calculator";

test("adds two numbers", () => {
    expect(add(2, 3)).toBe(5);
});