Level 1: Unit/Logic Tests

Pure JavaScript testing with vitest or jest for logic, data transforms, and state.

What Level 1 Tests

Level 1 tests verify pure logic -- functions that take inputs and return outputs without touching the DOM, browser APIs, or visual rendering.

Typical targets:

• Utility functions (string manipulation, date formatting, math)

• Data transforms (API response mapping, normalization)

• State reducers and selectors

• Validation logic

• Business rules

Tools

Example

// utils/format-price.ts
export function formatPrice(cents: number): string {
  return `$${(cents / 100).toFixed(2)}`;
}

// utils/format-price.test.ts
import { describe, it, expect } from "vitest";
import { formatPrice } from "./format-price";

describe("formatPrice", () => {
  it("formats cents to dollar string", () => {
    expect(formatPrice(1299)).toBe("$12.99");
  });

  it("handles zero", () => {
    expect(formatPrice(0)).toBe("$0.00");
  });

  it("handles single-digit cents", () => {
    expect(formatPrice(5)).toBe("$0.05");
  });
});

TDD Cycle

For logic changes, follow the standard TDD cycle:

1. Write a failing test that describes the expected behavior

2. Implement the minimum code to make the test pass

3. Verify the test is green

4. Refactor if needed

5. Repeat for each behavior

Blind Spots

Level 1 is the right choice when you are confident the bug is in logic, not in rendering. If there is any doubt about visibility or visual correctness, escalate to Level 4 or 5.

When to Use Level 1