js 判断是否存在

if (typeof myVariable !== 'undefined') {
    console.log('myVariable exists');
} else {
    console.log('myVariable does not exist');
}

const obj = { key1: 'value1' };

// 使用 in 操作符
if ('key1' in obj) {
    console.log('key1 exists in obj');
}

// 使用 hasOwnProperty 方法
if (obj.hasOwnProperty('key1')) {
    console.log('key1 exists in obj');
}

const arr = [1, 2, 3, 4, 5];

if (arr.includes(3)) {
    console.log('3 exists in arr');
} else {
    console.log('3 does not exist in arr');
}

if (typeof myFunction === 'function') {
    myFunction();
} else {
    console.log('myFunction does not exist');
}

const element = document.getElementById('myElement');

if (element) {
    console.log('#myElement exists in the DOM');
} else {
    console.log('#myElement does not exist in the DOM');
}