# E0379: optional parameter cannot be followed by a required parameter

Optional parameters need to be placed after the required ones.

    function f(param1?: number, param2: number) {
      if (param1) {
        return param1
      }
      return param2;
    }

To fix this warning, we need to place the required parameter(s) first, followed by the optional parameters

    function f(param2: number, param1?: number) {
      if (param1) {
        return param1
      }
      return param2;
    }

Documentation for other errors