# Combinations

Sometimes we allow function to accept different sequences of types. Let’s take an [example](https://github.com/npm/cli/blob/v6.9.0/lib/fetch-package-metadata.js):

```javascript
function andLogAndFinish( spec, tracker, done ) {
  validate( "SOF|SZF|OOF|OZF", [ spec, tracker, done ] )
  //...
}
```

Where the following sequences of types valid:

* string, object, function
* string, null, function
* object, object, function
* object, null, function

```javascript
import { validateCombo } from "bycontract";

const CASE1 = [ "string", TRACKER_OPTIONS, "function" ],
      CASE2 = [ "string", null, "function" ],
      CASE3 = [ SPEC_OPTIONS, TRACKER_OPTIONS, "function" ],
      CASE4 = [ SPEC_OPTIONS, null, "function" ];

validateCombo( arguments, [ CASE1, CASE2, CASE3, CASE4 ] );
```

Function `validateCombo` throws exception when none of the cases is valid