[解決済み] JavaScriptのオブジェクトにキーが存在するかどうかをチェックする？

var obj = { key: undefined };
console.log(obj["key"] !== undefined); // false, but the key exists!

var obj = { key: undefined };
console.log("key" in obj); // true, regardless of the actual value

var obj = { not_key: undefined };
console.log(!("key" in obj)); // true if "key" doesn't exist in object
console.log(!"key" in obj);   // Do not do this! It is equivalent to "false in obj"

var obj = { key: undefined };
console.log(obj.hasOwnProperty("key")); // true