# Evt.factorize(evt)

{% hint style="info" %}
This is the identity function with special type annotations.
{% endhint %}

If you have a variable that is either an `Evt` that post `A` or an `Evt` that post `B` you have an event that post `A or B`.

In other words `Evt<A> | Evt<B>` is assignable to `Evt<A | B >.` This method implements this property.

```typescript
import { Evt, VoidEvt, matchVoid } from "evt";

declare evt: Evt<string> | Evt<number> | VoidEvt = Evt.create<any>();

evt.attach(data=> { }); // TS ERROR

Evt.factorize(evt) // OK, return Evt<string | number | void>
    .attach(data=> { // data is string | number | void

        //To test if data is void
        if( matchVoid(data) ){
            return;
        }

        //Here data is string | number.

    })
    ;
```

See also [`FactorizeEvt<E>`](https://docs.evt.land/api/helpertypes#swapevttype-less-than-e-t-greater-than), helper type that this method levrage.


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