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

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

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

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