Day 45 of 100: Utility Types — Partial, Pick, Omit for Test Variants

Day 45 of 100 — 100 Days of Playwright with TypeScript Challenge. Phase: TypeScript. JavaScript (Days 1-33) → TypeScript (Days 34-63) → Playwright (Days 64-96) → AI Framework Capstone (Days 97-100).

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Utility types transform existing types. Test data variants without redefining shapes.

The Big Four

type User = { id: number; name: string; email: string; role: string };

// Partial — everything optional → override objects
type UserOverrides = Partial;

// Pick — subset of fields
type UserPreview = Pick;

// Omit — everything except
type NewUser = Omit;          // creation payload — no id yet

// Required — strip all optionality
type CompleteUser = Required;

The Factory Pattern These Enable

function makeUser(overrides: Partial = {}): User {
  return {
    id: Date.now(),
    name: 'Default Tester',
    email: `qa-${Date.now()}@test.com`,
    role: 'viewer',
    ...overrides,                       // spread — Day 14, now typed
  };
}

const admin = makeUser({ role: 'admin' });
const noEmail = makeUser({ email: '' });
// makeUser({ rol: 'admin' })  — error: typo caught by Partial

Readonly and Record Preview

type FrozenConfig = Readonly<{ baseUrl: string }>;
const cfg: FrozenConfig = { baseUrl: 'http://localhost' };
// cfg.baseUrl = 'x'  — error: readonly

type EnvUrls = Record<'local' | 'staging' | 'prod', string>;  // Day 46

Tomorrow (Day 46): Record and Readonly — typed lookup tables.