typescript中可选参数的语义

function greet(name: string, age?: number) {
  console.log(`Hello, ${name}!`);
  if (age) {
    console.log(`You are ${age} years old.`);
  }
}

greet("Alice"); // Output: Hello, Alice!
greet("Bob", 25); // Output: Hello, Bob! You are 25 years old.

function greet(name: string, age: number = 18) {
  console.log(`Hello, ${name}!`);
  console.log(`You are ${age} years old.`);
}

greet("Alice"); // Output: Hello, Alice! You are 18 years old.
greet("Bob", 25); // Output: Hello, Bob! You are 25 years old.

function greet(name?: string, age: number) {
  // Invalid function declaration
}