When doing a JavaScript transformation the transformedObject always represents the final state of the object when you are POSTing or GETting. 

For example, let's say I am starting with this originalObject: 

{  
  "Email":"test@cloud-elements.com",
  "phone":[  
    {  
      "number":"5555551234"
    },
    {  
      "number":"5558492927"
    }
  ],
  "FirstName":"hello",
  "LastName":"world"
}

My mapping looks like:

Now, in my transformation, I want to take the first phone number in the array and use it as the Phone field. Your JS should look something like this: 

if (!fromVendor) {
  if (originalObject.phone.length > 0) {
    transformedObject.Phone = originalObject.phone[0].number
  }
}
done(transformedObject);

Then let's say I want to go the other way. When I call GET /MyContacts I want the phone numbers in an array. 

Your JS should look like this: 

if (fromVendor) {
  if (originalObject.Phone) {
    transformedObject.phone = [];
    transformedObject.phone.push({number: originalObject.Phone});
  }
}
done(transformedObject);

Putting it all together to account for both directions: 

if (!fromVendor) {
  if (originalObject.phone.length > 0) {
    transformedObject.Phone = originalObject.phone[0].number
  }
} else {
  if (originalObject.Phone) {
    transformedObject.phone = [];
    transformedObject.phone.push({number: originalObject.Phone});
  }
}
done(transformedObject);