Day 38 of 100: Union and Intersection Types — Modeling Test States
Day 38 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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Unions say ‘one of these’. Intersections say ‘all of these’. Test states and combined configs, modeled precisely.
Contents
Unions
type TestStatus = 'passed' | 'failed' | 'skipped' | 'flaky';
type Id = string | number;
function report(status: TestStatus): string {
// switch over a union — compiler ensures all cases handled
switch (status) {
case 'passed': return 'green';
case 'failed': return 'red';
case 'skipped': return 'gray';
case 'flaky': return 'orange';
}
}
// report('error') — compile error: not in union🚀 Level Up Your Playwright
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Intersections
type Timestamps = { createdAt: string; updatedAt: string };
type WithId = { id: number };
type DbUser = WithId & Timestamps & { name: string };
// Must have ALL: id, createdAt, updatedAt, nameDiscriminated Unions — The Power Pattern
type ApiResult =
| { ok: true; data: { id: number } }
| { ok: false; error: string };
function handle(res: ApiResult) {
if (res.ok) {
res.data.id; // TS knows data exists here
} else {
res.error; // and error exists here — no casts needed
}
}Tomorrow (Day 39): Enums and literal types — when each wins.
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