javascript - Check if a key exists inside a JSON object

#short
#javascript
#json
amt: "10.00"
merchant_id: "sam"
mobileNo: "9874563210"
orderID: "123456"
passkey: "1234"

The above is the JSON object I'm dealing with. I want to check if the merchant_id key exists. I tried the below code, but it's not working. Any way to achieve it?
script
{
var thisSession = JSON.parse('?php echo json_encode($_POST); ?');
if (!("merchant_id" in thisSession)==0)
{
// do nothing.
}
else
{
alert("yeah");
}
}
/script