null, undefined und strikte Nullprüfungen

Willkommen

null vs. undefined

let a: undefined = undefined;
let b: null = null;

Ohne strikte Nullprüfung

// strictNullChecks: false (unsafe default)
let name: string = null; // allowed — dangerous!

Strikte Nullprüfung aktivieren

// strictNullChecks: true
let name: string = null; // Error!
let maybeName: string | null = null; // OK

Optionale Eigenschaften

interface Config {
  host: string;
  port?: number; // number | undefined
}

const c: Config = { host: 'localhost' };
console.log(c.port?.toString()); // optional chaining

Nullable Typen eingrenzen

function greet(name: string | null): string {
  if (name === null) return 'Guest';
  return 'Hello, ' + name;
}

Optional Chaining (?.)

const user = getUser();
const city = user?.address?.city; // undefined if any step is null

Nullish Coalescing (??)

const port = config.port ?? 3000; // 3000 if port is null/undefined
const label = name ?? 'Anonymous';

Non-Null-Assertion-Operator (!)

const el = document.getElementById('root')!;
el.textContent = 'Loaded'; // no null check needed

null vs. undefined zurückgeben

function findUser(id: number): User | null {
  return db.find(u => u.id === id) ?? null;
}

strictNullChecks verhindert den Milliarden-Dollar-Fehler

Schnelltest

Zusammenfassung