TS

// A discriminated union: the `kind` field is what lets the compiler
// tell the members apart.
type Result =
  | { kind: 'ok'; value: number }
  | { kind: 'error'; message: string };

function describe(result: Result): string {
  // Inside each branch the type is narrowed, so .value and .message
  // are both safe - and reaching for the wrong one is an error.
  switch (result.kind) {
    case 'ok':    return `Got ${result.value}`;
    case 'error': return `Failed: ${result.message}`;
  }
}

console.log(describe({ kind: 'ok', value: 42 }));
console.log(describe({ kind: 'error', message: 'nope' }));

// A generic that keeps the element type all the way through.
function first<T>(items: readonly T[]): T | undefined {
  return items[0];
}

const name = first(['Aisha', 'Kenji']);   // string | undefined
const score = first([92, 78]);            // number | undefined
console.log(name?.toUpperCase(), score?.toFixed(1));

// strictNullChecks is on, so this is an error rather than a crash:
//   console.log(name.toUpperCase());

const lengths = ['one', 'three', 'five'].map((s) => s.length);
console.log('Lengths:', lengths);

Output

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