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

@static
@memberOf _
@category Utilities
@param {String} [prefix] The value to prefix the id with.
@returns {Number|String} Returns a numeric id if no prefix is passed, else
 a string id may be returned.
@example

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