JS中的undefined 与 null

let x;
console.log(x); // undefined

let obj = { name: "John", age: 30 };
console.log(obj.address); // undefined

let arr = [1, 2, 3];
console.log(arr[3]); // undefined

function foo() {
// 没有明确返回值
}

console.log(foo()); //undefined

let x = null;
console.log(x); // null

let obj = null; // 初始化为 null
obj = { name: "John", age: 30 }; // 后续设置为对象

function foo(arg) {
  if (arg === null) {
    console.log("参数为空");
  } else {
    console.log("参数不为空");
  }
}

foo(null); // 参数为空
foo("Hello"); // 参数不为空

console.log(undefined == null); // true
console.log(undefined === null); // false

let x;
console.log(typeof x); // "undefined"

let y = null;
console.log(typeof y); // "object"

function foo(x = "default") {
    console.log(x);
}

foo(); // "default"
foo(undefined); // "default"
foo(null); // null

let obj = { name: "John", address: { city: "New York" } };

console.log(obj.address?.city); // "New York"
console.log(obj.address?.zipCode); // undefined

let x = undefined;
console.log(x); // undefined

let y = null;
console.log(y); // null