how to add key in an object after checking if it available or not code example

Example 1: how to check if a key exists in an object javascript

!("key" in obj) // true if "key" doesn't exist in object
!"key" in obj   // ERROR!  Equivalent to "false in obj"

Example 2: js check if object key exists

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