checking if a key exists in a javascript object code example

Example 1: javascript does key exist

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

Example 2: check if a key exists in an object javascript

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

Example 3: js check if object key exists

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