how to check if a element is in object in javascript code example

Example 1: javascript does key exist

var person={"name":"Billy","age":20}
person.hasOwnProperty("name"); // true
person.hasOwnProperty("sex"); // false

Example 2: js check if object key exists

var obj = { key: undefined };
obj["key"] !== undefined // false, but the key exists!

Example 3: js key in dict

"key" in obj // true, regardless of the actual value