35. Testing typed code

Unit tests for TypeScript run against the compiled JavaScript, so a testing framework like Vitest or Jest exercises runtime behaviour exactly as JS would, while `tsc`/the editor catches type errors separately, before tests even run.

ts
function calculateFee(amountWei: bigint, bps: number): bigint {
  return (amountWei * BigInt(bps)) / 10000n;
}

// example test (Vitest-style)
import { describe, it, expect } from "vitest";

describe("calculateFee", () => {
  it("charges the right basis points", () => {
    expect(calculateFee(1_000_000n, 50)).toBe(5000n);
  });
});

Good tests for typed code focus on behaviour and edge cases (zero amounts, negative inputs, boundary values) rather than re-testing what the compiler already guarantees (like argument types). Type-level testing tools (`expectTypeOf`, `tsd`) exist for library authors who want to assert that generic inference produces the expected type.

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