function myFunc( requiredA, requiredB,  optionalA, optionalB, optionalC ) {

​

  switch (arguments.length - 2) { // 2 is number of required arguments

    case 0:  optionalA = 'Some default';

    case 1:  optionalB = 'Another default';

    case 2:  optionalC = 'Some other default';

  }

​

  console.log( requiredA, requiredB, optionalA, optionalB, optionalC );

}

​

myFunc( 'test', 1 );

​

myFunc( 'test', 2, 'override' );

​

myFunc( 'test', 3, 'override', false );

​

myFunc( 'test', 4, 'override', false, undefined );