JavaScript делает ключ
var person={"name":"Billy","age":20}
person.hasOwnProperty("name"); // true
person.hasOwnProperty("sex"); // false
Grepper
var person={"name":"Billy","age":20}
person.hasOwnProperty("name"); // true
person.hasOwnProperty("sex"); // false"key" in obj // true, regardless of the actual value!("key" in obj) // true if "key" doesn't exist in object
!"key" in obj // ERROR! Equivalent to "false in obj"
"key" in obj // true, regardless of the actual value
var obj = { key: undefined };
obj["key"] !== undefined // false, but the key exists!