angular2中未定义的值

let name: string;
console.log(name); // 输出 undefined

let person = { name: 'John', age: 25 };
console.log(person.address); // 输出 undefined

function greet(): void {
  console.log('Hello!');
}
let result = greet();
console.log(result); // 输出 undefined

if (typeof name !== 'undefined') {
  console.log(name);
} else {
  console.log('Name is undefined');
}

let age: number;
let defaultAge = 18;
console.log(age || defaultAge); // 如果age是未定义的，则输出默认值18