Customizing the Fetcher and Default Client Options

You can redefine the default fetching function and its options to tightly integrate Vovk.ts client with your application state or to add extra features. For example, the clientized controller methods may look like that:

import { UserController } from 'vovk-client';
// ...
    // custom options
    successToast: 'Successfully created a new user',
    useAuth: true,
    sentryLogErrors: true,

The fetcher is defined as a default export that extends VovkClientFetcher type and should be listed either as config option:

/** @type {import('vovk').VovkConfig} */
const vovkConfig = {
    fetcher: './src/lib/myFetchingFunction',
module.exports = vovkConfig;

Or as VOVK_FETCHER env variable:

VOVK_FETCHER="./src/lib/myFetchingFunction" vovk dev

By default Vovk.ts uses fetcher defined at vovk/client/defaultFetcher and you can check its source code on Github (opens in a new tab).

The fetcher accepts two arguments:

  • An object that is provided by the internal Vovk.ts code that includes HTTP method information and utilities:
    • httpMethod - the HTTP metod;
    • getEndpoint - an utility that builds request endpoiint from prefix, query and params;
    • validate - a function that validates body and query of the request;
    • defaultHandler - handles the Response object returned from fetch function;
    • defaultStreamHandler - handles the Response object returned from fetch function in case of a stream.
  • Request arguments:
    • params - the patams such as id from users/:id;
    • query - the search query properties such as ?foo=bar;
    • body - the request body;
    • prefix - what's defined as prefix property at vovk.config.js or passed directly to the client method;
    • The rest options - your custom options and RequestInit (including custom Next.js options (opens in a new tab)) that includes the rest fetch options such as headers, credentials etc.

Your custom fetcher with a custom option successMessage may look like that:

import type { VovkDefaultFetcherOptions, VovkClientFetcher } from 'vovk';
// in order to keep default features such as disableClientValidation, headers etc,
// it's recommended to extend custom options from VovkDefaultFetcherOptions
interface MyOptions extends VovkDefaultFetcherOptions {
    successMessage: string;
const myFetchingFunction: VovkClientFetcher<MyOptions> = async (
  { httpMethod, getEndpoint, validate, defaultHandler, defaultStreamHandler },
  { params, query, body, prefix = '/api', successMessage, ...options }
) => {
  // 1. Build the endpoint
  const endpoint = getEndpoint({ prefix, params, query });
  // 2. Validate
  if (!options.disableClientValidation) {
    await validate({ body, query });
  // 3. Make fetch request (here you can add authorisation headers)
  const response = await fetch(endpoint, {
    method: httpMethod,
    body: JSON.stringify(body),
  let returnResponse = response;
  // 4. Handle response based on response headers
  if (response.headers.get('content-type')?.includes('application/json')) {
    returnResponse = await defaultHandler(response);
  } else if (response.headers.get('content-type')?.includes('text/event-stream')) {
    returnResponse = await defaultStreamHandler(response);
  // 5. Utilise your custom option somehow.
  return returnResponse;
export default myFetchingFunction;

As you can see the code determines response type by content-type header. You can freely redefine this logic to make the fetcher return something else (for example, if you want to use Axios or other fetching library).

if (response.headers.get('content-type')?.includes('application/json')) {
  return yourCustomHandler(response);

In case if the server endpoint and yourCustomHandler return different values, you can redefine the inferred return type using the client method generic argument.

import { MyController } from 'vovk-client';
// ...
const result = await MyController.myMethod<{ foo: 'bar' }>({
  successMessage: 'Success!'

The result variable from this example is going to receive { foo: 'bar' } type.

Creating a custom validation library

If you need to create your custom validation library, check decorators documentation.