js怎样判断undefine

let myVariable;

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

let myVariable;

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

let myVariable;

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

function checkUndefined(value) {
    if (typeof value === 'undefined') {
        console.log('The value is undefined');
    } else {
        console.log('The value is defined:', value);
    }
}

let a;
checkUndefined(a); // 输出: The value is undefined

let b = void 0;
checkUndefined(b); // 输出: The value is undefined

let c = 'Hello';
checkUndefined(c); // 输出: The value is defined: Hello