การทำชนิดให้แคบลงด้วย typeof และค่าความจริง

function process(val: string | number) {
  if (typeof val === 'string') {
    return val.toUpperCase(); // val: string
  }
  return val.toFixed(2); // val: number
}

typeof 'hello'    // 'string'
typeof 42         // 'number'
typeof true       // 'boolean'
typeof null       // 'object' (quirk!)
typeof undefined  // 'undefined'

function greet(name: string | null) {
  if (name) {
    console.log('Hello, ' + name); // name: string
  } else {
    console.log('Hello, stranger');
  }
}

const items: (string | null)[] = ['a', null, 'b', null];
const strings = items.filter((x): x is string => x !== null);

function getLength(val: string | null): number {
  if (!val) return 0; // early return narrows
  return val.length;  // val: string here
}

function accept(x: string | number | boolean) {
  if (typeof x === 'string' || typeof x === 'number') {
    console.log(x); // x: string | number
  }
}

function check(x: string | null | 0) {
  if (x === null) { /* x: null */ }
  else if (x === 0) { /* x: 0 */ }
  else { /* x: string */ }
}

let x: string | number = 'hello';
// x: string
x = 42;
// x: number