js 变量是否为空

let variable;

if (variable === undefined || variable === null) {
    console.log('变量为空');
} else {
    console.log('变量不为空');
}

let variable = '';

if (variable === '') {
    console.log('变量为空字符串');
} else {
    console.log('变量不为空字符串');
}

let variable = 0;

if (!variable) {
    console.log('变量为空或为假值');
} else {
    console.log('变量不为空且为真值');
}

function isEmpty(variable) {
    return variable === undefined || variable === null || variable === '';
}

let variable1;
let variable2 = null;
let variable3 = '';

console.log(isEmpty(variable1)); // true
console.log(isEmpty(variable2)); // true
console.log(isEmpty(variable3)); // true
console.log(isEmpty(123)); // false