js判断为undefined

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

let obj = {};
console.log(obj.nonExistentProperty); // undefined

if (typeof a === 'undefined') {
    console.log('变量 a 未定义');
}

if (obj.nonExistentProperty === undefined) {
    console.log('对象 obj 的 nonExistentProperty 属性不存在');
}

function doSomething() {
    // 没有 return 语句
}

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

function doSomethingElse() {
    return 'Hello, World!';
}

console.log(doSomethingElse()); // 'Hello, World!'

let arr = [];
console.log(arr[0]); // undefined

arr[0] = 'Hello';
console.log(arr[0]); // 'Hello'

// 变量检查
let x;
if (typeof x === 'undefined') {
    x = '默认值';
}
console.log(x); // '默认值'

// 对象属性检查
let person = {};
if (person.name === undefined) {
    person.name = '匿名';
}
console.log(person.name); // '匿名'

// 函数返回值检查
function getAge() {
    // 忘记返回值
}

let age = getAge();
if (age === undefined) {
    age = 0;
}
console.log(age); // 0