Uncaught TypeError: Cannot use 'in' operator to search for '' in JSON string

You may get this error if you are using a string as an array. Say that if you got a json from an ajax, and you forgot to parse the result, and using the result as an array. The remedy is as above, to parse the json before using it.

You need to parse the string in your populateValue variable to an object:

prePopulate: $.parseJSON(populateValue)

Or alternatively, in plain JS:

prePopulate: JSON.parse(populateValue)