OK, looks like I had the right answer unless if you don't want inherited properties:

if (x.hasOwnProperty('key'))

Here are some other options to include inherited properties:

if (x.key) // quick and dirty but does the same thing as below.

if (x.key !== undefined)

OK, it looks like I had the right answer unless if you don't want inherited properties:

if (x.hasOwnProperty('key'))

Here are some other options to include inherited properties:

if (x.key) // Quick and dirty, but it does the same thing as below.

if (x.key !== undefined)

OK, looks like I had the right answer unless if you don't want inherited properties:

if (x.hasOwnProperty('key'))

Here are some other options to include inherited properties:

if (x.key) // quick and dirty but does the same thing as below.

if (x.key !== 'undefined')

OK, looks like I had the right answer unless if you don't want inherited properties:

if (x.hasOwnProperty('key'))

Here are some other options to include inherited properties:

if (x.key) // quick and dirty but does the same thing as below.

if (x.key !== undefined)