---
id: data-transformers
title: Data Transformers
sidebar_label: Data Transformers
slug: /server/data-transformers
---

You are able to serialize the response data & input args. The transformers need to be added both to the server and the client.

## Using [superjson](https://github.com/blitz-js/superjson)

SuperJSON allows us to transparently use, e.g., standard `Date`/`Map`/`Set`s over the wire between the server and client. That is, you can return any of these types from your API-resolver and use them in the client without having to recreate the objects from JSON.

### How to

#### 1. Install

```bash
yarn add superjson
```

#### 2. Add to your `initTRPC`

```ts title='routers/router/_app.ts'
import { initTRPC } from '@trpc/server';
import superjson from 'superjson';

export const t = initTRPC.create({
  transformer: superjson,
});
```

#### 3. Add to `httpLink()`, `wsLink()`, etc

> TypeScript will guide you to where you need to add `transformer` as soon as you've added it on the `initTRPC`-object

`createTRPCClient()`:

```ts title='src/app/_trpc/client.ts'
import { createTRPCClient } from '@trpc/client';
import type { AppRouter } from '~/server/routers/_app';
import superjson from 'superjson';

export const client = createTRPCClient<AppRouter>({
  links: [
    httpLink({
      url: 'http://localhost:3000',
      // transformer: superjson
    }),
  ],
});
```

## Different transformers for upload and download

If a transformer should only be used for one direction or different transformers should be used for upload and download (e.g., for performance reasons), you can provide individual transformers for upload and download. Make sure you use the same combined transformer everywhere.

### How to

Here [superjson](https://github.com/blitz-js/superjson) is used for uploading and [devalue](https://github.com/Rich-Harris/devalue) for downloading data because devalue is a lot faster but insecure to use on the server.

#### 1. Install

```bash
yarn add superjson devalue
```

#### 2. Add to `utils/trpc.ts`

```ts title='utils/trpc.ts'
import { uneval } from 'devalue';
import superjson from 'superjson';

// [...]

export const transformer = {
  input: superjson,
  output: {
    serialize: (object) => uneval(object),
    // This `eval` only ever happens on the **client**
    deserialize: (object) => eval(`(${object})`),
  },
};
```

#### 3. Add to your `AppRouter`

```ts title='server/routers/_app.ts'
import { initTRPC } from '@trpc/server';
import { transformer } from '../../utils/trpc';

export const t = initTRPC.create({
  transformer,
});

export const appRouter = t.router({
  // [...]
});
```

## `DataTransformer` interface

```ts
export interface DataTransformer {
  serialize(object: any): any;
  deserialize(object: any): any;
}

interface InputDataTransformer extends DataTransformer {
  /**
   * This function runs **on the client** before sending the data to the server.
   */
  serialize(object: any): any;
  /**
   * This function runs **on the server** to transform the data before it is passed to the resolver
   */
  deserialize(object: any): any;
}

interface OutputDataTransformer extends DataTransformer {
  /**
   * This function runs **on the server** before sending the data to the client.
   */
  serialize(object: any): any;
  /**
   * This function runs **only on the client** to transform the data sent from the server.
   */
  deserialize(object: any): any;
}

export interface CombinedDataTransformer {
  /**
   * Specify how the data sent from the client to the server should be transformed.
   */
  input: InputDataTransformer;
  /**
   * Specify how the data sent from the server to the client should be transformed.
   */
  output: OutputDataTransformer;
}
```
