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2.1.1 • Public • Published

Simple HTTP REST Client

This library provides a framework for creating a REST HTTP client in TypeScript and JavaScript. In other words, this library can be used to consume every type of API that uses the request/response pattern. But this library should not be used to consume websockets or GraphQL: workflows involved in these cases are specific, there is no point in trying to unite them with what is being done in this library.

This library implements Simple HTTP Request Builder with fetch.

Though this library works seamlessly with Jersey module of Plume Framework, it can be easily adapted to work with any HTTP API.

This library is used by default in the project templates provided by Create Plume React Project.


Using NPM:

npm install simple-http-rest-client

Using Yarn:

yarn add simple-http-rest-client

Main components example

To consume an API, there are 3 main steps:

  1. Configure the API client: this client will build a request and with the attached HTTP client. The API client will generally be the same for all endpoints of the same host. More API clients may be needed to differentiate public and authenticated parts of the whole host API catalog.
  2. Configure each API endpoint
  3. Consume the API endpoint

Configure the API client

This function be called for each API endpoint identified by a Method and a Path. A request builder will be returned, when executed, this request will return a HttpPromise.

restRequest<T>(method: HttpMethod, path: string): HttpRequest<HttpPromise<T>> {
  return createHttpFetchRequest(
    baseUrl, // the base URL, e.g.
    method, // the method, e.g. HttpMethod.GET
    path, // the path, e.g. /users/123/addresses
    defaultJsonFetchClient, // the base API client
    options, // optional http options

In a browser environment, the base URL can be built using the window.location object: ${window.location.protocol}//${}/api

Configure each API endpoint

Here the goal is to configure the specific API call to an endpoint. So the previous API client created will be used.

In the following example:

  • The endpoint path is /admin/session to authenticate a user by verifying its credentials
  • This endpoint takes a JSON object as a body argument that complies to the TS type SessionCredentials
  • This endpoint returns a JSON object that complies to the TS type SessionToken
authenticate(credentials: SessionCredentials): HttpPromise<SessionToken>  {
  return httpClient
    .restRequest<SessionToken>(HttpMethod.POST, '/admin/session')
    // this line will execute the request on the httpClient and return the result promise

Consume the API endpoint

Here the previous configured API endpoint call will be used to actually make the call inside the project logic.

authenticate(credentials: SessionCredentials) {
    .then((sessionToken: SessionToken) => {
      console.log(`Session created ${sessionToken.webSessionToken}, should grant access to the user`);
    .catch((httpError: HttpError) => {
      // see below to have a look of the error codes handled by the library and how to configure you owns errors
      console.log(`Authentication failed, error code: ${httpError.errorCode}`);

Types and main concepts

Basic types are:

On top of that, the HttpPromise is often used on a HttpResponse object to:

  • Ease the usage of the Promise
  • Make sure to have logs in case an error occurred even though a catch statement is not set


It represents a request that will be executed by an HttpClient. This is the main object defined in Simple HTTP Request Builder. See directly the source type for advanced documentation:


This object represents the successful or failed HTTP response. It is returned by an HttpClient and by a FetchResponseHandler. HTTP failures are represented by an error. See directly the source type for advanced documentation:


Handlers are executed after a successful HTTP response is available: this means an HTTP response has been received (whichever the response statut, 200, 400 or 500...). These handlers will:

  • Validate some preconditions and if necessary return an error result
  • Return a result

So a handler can:

  • Either return a result (which can be a successful result or an error), in that case next handlers will not be executed
  • Either return undefined, in that case next handlers will be executed

Expected results should be of type Promise of HttpResponse.

Here are two samples usage:


A mutable safe Promise that ensures:

  • Either a then function is provided or either an info log statement will be issued
  • Either a catch function is provided or either a warning log statement will be issued
  • Then and Catch functions are wrapped to ensure that if an error occurs during the execution, an error statement is issued

See directly the source type for advanced documentation:


Http errors can be:

  • Raised by the library, in that case the work expected in a project will only be to display the correct error message to the end user if necessary
  • Raised by the project that configured the library to a specific API that raises its own errors

See directly the source type for advanced documentation:

Errors raised by the library

The errors handled by the library are:

  • NETWORK_ERROR: It means the remote API could not be contacted, it is likely due to poor network connection on the client side
  • TIMEOUT_ERROR: It means the remote API could be contact but no result has been returned after the timeout delay. It might also be due to poor network connection, but it can also be due to an API issue. The default timeout is 20 seconds, but that can be configured
  • FORBIDDEN_ERROR: It means the API returned a 403 response. This error is raised by the validator validateBasicStatusCodes
  • INTERNAL_ERROR: It means a parsing error occurred: a then function provided to a HttpPromise.then() function raised an error, the parsing of the JSON object raised an error, the server returned a non JSON response, etc. Since this error is related to a developer error or a backend error, we generally want to display the same generic error to an end user. If this error is not suitable for a project, it is possible to get rid of this by having customized validator and wrapping then and catch functions provided to HttpPromise to make sure these functions never fails or raised custom HttpError

These errors are already mapped to default messages in the Create Plume React Project templates:

Step by step custom usage

Here are the steps to use any custom API:

Create a basic API client

Start by creating a proxy to the fetchClient method:

const apiFetchClient = <T>(httpRequest: HttpRequest<unknown>): Promise<HttpResponse<T>> => fetchClient(httpRequest);

Note that the function fetchClient takes rest parameters/varargs of type FetchResponseHandler. These parameters enable to customize the client. This is covered in the next following sections:

Add validators

The goal here is to verify that the result is excepted and correct.

By default, these validators are provided:

  • validateBasicStatusCodes: It raises an error if the status code is 403, and it returns an empty response if the status code is 200
  • jsonContentTypeValidator: It verifies the response content-type is JSON
  • contentTypeValidator: It is used to create response content-type validator like jsonContentTypeValidator

Once validators are added, our API client should look like this:

const apiFetchClient = <T>(httpRequest: HttpRequest<unknown>): Promise<HttpResponse<T>> => fetchClient(
  // here are the response validators for our custom API
  validateBasicStatusCodes, jsonContentTypeValidator,

Add a response mapper

The goal here is to parse the response's body and return the correct object. For example to parse a JSON content:

  1. The JSON body should be parsed (using response.json())
  2. Depending on the status code:
    1. If the status code is 2xx, the response object should be appended to the field HttpResponse.response
    2. Else the response is an error, the error might be categorized, it should then be appended to the field HttpResponse.error
  3. If the JSON parsing failed, an error should be to the field HttpResponse.error

All these actions are implemented in the function toJsonResponse(response: Response, jsonErrorMapper: JsonErrorMapper = defaultJsonErrorMapper): Promise<HttpResponse<unknown>>:

  • Parameter response is the fetch Response to parse
  • Parameter jsonErrorMapper is the JsonErrorMapper that will handle the parsed JSON object in case the HTTP response is not successful: i.e. status code is not 2xx

So to configuring the JSON response mapper for a dedicated API should look like this:

// First the error mapper should be configured
// Here what is important is to create errors that will be handled later:
//  either for processing or for displaying to the user
// So if an API should be consumed and the errors are more or less ignored,
//  the implementation should contain a simple logger statement with a genericError
const apiJsonErrorMapper: JsonErrorMapper = (response: Response, json: any) => {
  // The mapping can be done using the response object
  if (response.status === 401) {
    // a custom error that could be created for the need of the project
    return insufficientPermissionsError;
  // The mapping can be done using the JSON response body object
  if (typeof json.errorCode !== 'undefined') {
    return { error: json };
  // If the error is not recognized a generic error should be issued
  logger.error('Unrecognized JSON error', response);
  return { error: genericError };

Once this error mapper is configured, the response mapper function can be configured:

// The response mapper is creating by configuring toJsonResponse with our custom apiJsonErrorMapper
const toApiJsonResponse
: FetchResponseHandler = (response) => toJsonResponse(response, apiJsonErrorMapper);

Finally, the API client can be configured with the response mapper:

const apiFetchClient = <T>(httpRequest: HttpRequest<unknown>): Promise<HttpResponse<T>> => fetchClient(
  // here are the response validators for our custom API
  validateBasicStatusCodes, jsonContentTypeValidator,
  // the response mapper

This can then be used following the main components example described above by replacing defaultJsonFetchClient by apiFetchClient.

Note that if an API is using XML or any other language, the method toJsonResponse should be rewritten to accept this specific language.

Advanced usages

Some components that can be used for specific use cases are described here.


The use case addressed here is: How to avoid calling multiple times the same API whereas the first call has not yet been resolved. So for example:

  • Multiple components need to get a configuration value that is available through an API
  • The result of the API is put in cache
  • But by default, when loading the application, the API will be called multiple times because the first call did not yet resolve

The SynchronizedHttpPromise will ensure there is only one HttpPromise running at a time.


This class enables to check what are the status of promises that are being executed.

A use case is Server-Side-Rendering:

  • After a first render
  • It is interesting to watch the promises that are being executed
  • And when all promises have revolved, it is guessable that all the data are ready for the application to rerender again

For more information, see the corresponding source code:

When used with an HttpFetchClient, HttpPromiseMonitor should be used instead of PromiseMonitor. It can then be used like that:

restRequest<T>(method: HttpMethod, path: string): HttpRequest<HttpPromise<T>> {
  return createHttpFetchRequest(
    this.httpPromiseMonitor.makeMonitor(defaultJsonFetchClient, { method, path }),

Tree shaking

This library supports tree shaking: components from this library that are not used will not end in your final build as long as your bundler supports this feature. For example, if the component SynchronizedHttpPromise is not used, it will not be packaged in your final application build.

Vite is a great bundler to achieve this.



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