Answers for "check if key exists in object and value is not flase in js"

javascript check if key exists in object

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

If you want to check if a key doesn't exist, remember to use parenthesis:
!("key" in obj) // true if "key" doesn't exist in object
!"key" in obj   // ERROR!  Equivalent to "false in obj"

Or, if you want to particularly test for properties of the object instance (and not inherited properties), use hasOwnProperty:
obj.hasOwnProperty("key") // true

check if key does not exists in dictionary javascript

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