js如何判断undefined

let myVariable;

if (myVariable === undefined) {
    console.log('myVariable is undefined');
}

let myVariable;

if (typeof myVariable === 'undefined') {
    console.log('myVariable is undefined');
}

let myVariable;

if (myVariable === void 0) {
    console.log('myVariable is undefined');
}

function processUser(user) {
    if (typeof user.name === 'undefined') {
        console.log('User name is missing');
    } else {
        console.log('User name:', user.name);
    }
}

let user1 = { name: 'Alice' };
let user2 = {};

processUser(user1); // 输出: User name: Alice
processUser(user2); // 输出: User name is missing