[Дзен-трансгуманист]

EcmaScript 6 Map

function uniquesOnly ( source ) {

  var uniqueness = new Map();

  source.forEach( function ( value ) {
    switch ( uniqueness.get( value )) {
      case undefined: uniqueness.set( value, true ); return;
      case true: uniqueness.set( value, false );
    }
  });

  return source.filter( function ( value ) {
    return uniqueness.get( value );
  });
}

alert(uniquesOnly([1, 2, 1, 10, 5, 3, 4, 40, 50]));