js length undefined

let arr = [1, 2, 3];
console.log(Array.isArray(arr)); // true
console.log(arr.length); // 3

let str = "Hello";
console.log(typeof str); // "string"
console.log(str.length); // 5

let notAnArray = { key: "value" };
console.log(Array.isArray(notAnArray)); // false
// console.log(notAnArray.length); // 这将返回 undefined

let obj = { key: "value" };

if (obj && typeof obj.length !== 'undefined') {
    console.log(obj.length);
} else {
    console.log("Object does not have a length property");
}

let length = (obj && typeof obj.length !== 'undefined') ? obj.length : 0;
console.log(length); // 0 if obj does not have length property