javascript if exists in object code example

Example 1: how to check if object exists in javascript

if (typeof maybeObject != "undefined") {
   alert("GOT THERE");
}

Example 2: 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 3: js check if object key exists

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