Actually, checking for undefined-ness is not an accurate way of testing whether a key exists. What if the key exists but the value is actuallyundefined?

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

You should instead use theinoperator:

"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), usehasOwnProperty:

obj.hasOwnProperty("key") // true