I got the below error while trying store contact form value to zoho. 

My xml code:

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When i give the name and email address as hard coded it will works fine:

var xmlDoc = '<Contacts><row no="1"><FL val="First Name">Peter</FL><FL val="Last Name">jhon</FL><FL val="Email">peter.r@gmail.com</FL></row></Contacts>';

While concatenating it throw error

Error:

<message> Unable to parse XML data </message>

var xml = '<Contacts><row no="1"><FL val="First Name">' + contact.firstName + '</FL><FL val="Last Name">' + contact.lastName + '</row></Contacts>';