# Your own types

In case you are using your own types or types from external library,
the SDK provides a way to convert the SDK types to and from your own
types.

The conversion is handled by "type handlers". You need to pass
`typeHandlers` when you create a new SDK instance.

A type handler is an object containing the following properties:

| Property | Description |
| -------- | ----------- |
| `sdkType` | The SDK type. |
| `appType` | The application specific type to convert SDK type to/from. |
| `canHandle` | A predicate function (i.e. function that returns truthy or falsy) that defines whether the this handler should be used. The SDK makes sure that `canHandle` will be called only for not `null` Object values, thus `null` check can be safely omitted.|
| `reader` | Conversion function. Gets an instance of `sdkType` as an argument, should return instance of `appType`. |
| `writer` | Conversion function. Gets an instance of `appType` as an argument, should return instance of `sdkType`. |

Either `appType` or `canHandle` should be provider, but not both.

Please note: v1.4.0 renamed `type` to `sdkType` and `customType` to `appType`.

**Example:** Convert
[`google.maps.LatLng`](https://developers.google.com/maps/documentation/javascript/reference/3/#LatLng)
to/from `LatLng`, [Decimal.js](https://github.com/MikeMcl/decimal.js/)
`Decimal` to/from `BigDecimal` and own plain object UUID
representation to UUID type class instance.

```js
const { BigDecimal, LatLng } = require('sharetribe-flex-sdk').types;

const sdk = createInstance({
  clientId: config.sdk.clientId,
  baseUrl: config.sdk.baseUrl,
  typeHandlers: [
    {
      sdkType: BigDecimal,
      appType: Decimal,
      writer: v => new BigDecimal(v.toString()),
      reader: v => new Decimal(v.value),
    },
      sdkType: LatLng,
      appType: google.maps.LatLng,
      writer: v => new LatLng(v.lat(), v.lng()),
      reader: v => new google.maps.LatLng(v.lat, v.lng)
    },
    {
      sdkType: UUID,
      canHandle: v => v.isMyOwnUuidType,
      writer: v => new UUID(v.myOwnUuidValue),
      reader: v => ({myOwnUuidValue: v.uuid,
                     isMyOwnUuidType: true})
    }
  ]
});
```

**Caveat:** Only body parameters are converted. An error *Don't know
how to serialize query parameter* will be thrown if query parameters
contain your own types.