Day 35 of 100: Basic Types — Annotations and Inference

Day 35 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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Annotate variables, parameters, and returns. The compiler enforces the rest.

Annotations

const name: string = 'Playwright';
const retries: number = 2;
const headless: boolean = true;
const browsers: string[] = ['chromium', 'firefox'];
const codes: Array = [200, 201, 204];   // same as number[]

function buildUrl(path: string): string {
  return 'http://localhost:3000' + path;
}

// buildUrl(42)        — error: number not assignable to string
// const x: string = 5 — error caught instantly

Inference — Let TS Work

const env = 'staging';        // inferred as string (actually 'staging' literal)
const timeout = 30_000;       // inferred number
const tags = ['smoke', 'ui']; // inferred string[]

// Rule: annotate function signatures and empty containers;
// let inference handle initialized locals.
const failures: string[] = [];   // empty array NEEDS annotation

Tomorrow (Day 36): Objects and type aliases — typing test data.