function uniqueId(prefix) {
      var id = ++idCounter;
      return String(prefix == null ? '' : prefix) + id;
    }
Generates a unique ID. If `prefix` is provided the ID is appended to it.

@static
@memberOf _
@category Utility
@param {string} [prefix] The value to prefix the ID with.
@returns {string} Returns the unique ID.
@example

_.uniqueId('contact_');
// => 'contact_104'

_.uniqueId();
// => '105'