rest-api-helper is a tiny lightweight package that abstracts the process of making HTTP requests.

Installation

Install the package using npm:

npm install rest-api-helper

or yarn:

yarn add rest-api-helper

Usage

To perform any request it is required to:

• Define transport aka the way you're going to communicate
• Configure client to glue everything together (base url, headers, transport etc)
• Create request object

---

Transport implementation

The Transport interface obliges you to implement one single method handle. It can do whatever you want whether it's fetch or XHR or setTimeout mock. In most cases, you're going to use the fetch API:

import { Transport } from "rest-api-helper";

const transport: Transport<Response> = {
  handle(request) {
    return fetch(request.url.href, request);
  }
};

---

Interceptor implementation (Optional)

The Interceptor interface binds you to implement onResponse method. Instead of being resolved immediately, original promise will fall through interceptor pipeline. Each onResponse call comes along with three arguments:

• request: Request – original request object
• response: T – received response
• promise: OriginalPromise<T> - original Promise handles (resolve and reject functions)

It allows you to intercept, analyze and modify responses before they are returned. This might be useful for scenarios like handling unauthorized responses or refreshing tokens:

import { Interceptor } from "rest-api-helper";

const interceptor: Interceptor<Response> = {
  onResponse: async (request, response, promise) => {
    if (response.ok) {
      promise.resolve(response); // bypass
      return;
    }

    if (response.status === 401) {
      // - refresh access token
      // - reattempt original request with new headers
      // - return new response
    }

    promise.reject(new Error("Unknown error"));
  }
};

---

Client Configuration

Create a new Client instance, configure it with a base URL, transport, interceptor (if needed) and deafault headers (if needed):

import { Client } from "rest-api-helper";

const client = new Client<Response>("https://api.frankfurter.app")
  .setTransport(transport)
  .setInterceptor(interceptor)
  .setDefaultHeaders({ "content-type": "application/json" });

---

Request Execution

Scaffold request and perform it (you can use predefined classes like Get, Post etc. or create it from scratch using Request):

import { Get, Request } from "rest-api-helper";

const get = new Get("/latest")
  .setSearchParam("amount", 10)
  .setSearchParam("from", "GBP")
  .setSearchParam("to", "USD");


const request = new Request("/latest", "get")
  .setSearchParam("amount", 10)
  .setSearchParam("from", "GBP")
  .setSearchParam("to", "USD");

const response = await client.perform(request);
const parsed = await response.json();

---

As you might have noticed Transport, Interceptor and Client have generic type arguments:

Transport<T>
Interceptor<T>
Client<T>

T defines the shape of each response. Since transport object responsible for performing requests, it dictates the response type. In order to be compatible, Transport, Interceptor and Client should share the same type.

In example described above, we used fetch API that is directly returned from handle method. Thus, generic type is native Response. However, we could easily move response parsing into the transport and replace native Response with something like this:

import { Transport, Request } from "rest-api-helper";

type CustomResponse = {
  data: unknown;
  status: number;
};

const transport: Transport<CustomResponse> = {
  async handle(request: Request) {
    const response = await fetch(request.url, request);
    const parsed = await reponse.json();

    return { data: parsed, status: response.status } as CustomResponse;
  }
};

// ...
// const interceptor: Interceptor<CustomResponse> = {...}
// const client = new Client<CustomResponse>(...)

API Reference

Request Class

Properties

• readonly url: URL
• readonly method: string
• readonly headers: Record<string, string>
• readonly isInterceptionAllowed: boolean
• readonly body: BodyInit | null

Constructor

constructor(path: string, method: string)

Creates a new request with a path and a method (GET, POST, PUT, DELETE etc.).

• path: a string that follows the base URL - /users. Can contain URL parameters, e.g. /users/:id
• method: a string that represents an HTTP method, e.g. GET, POST, PUT, DELETE. Case-insensitive.

Throws Error if path contains duplicate URL parameters. For example: /users/:id/devices/:id

Methods

setHeader(key: string, value: string): Request

Appends or overrides an existing header by key

• key: a header name, case-insensitive
• value: a header value

---

setHeaders(headers: Record<string, string>): Request

Merges passed record with existing one.

• headers: an object with key-value pairs, where key is a header name. Keys are case-insensitive

---

removeHeader(key: string): Request

Removes a header by key, if it exists.

• key: a header name, case-insensitive

---

setBody(data: BodyInit): Request

Sets the body of the request.

• data: the request body data

---

setBodyJSON(data: Record<string, unknown> | Record<string, unknown>[]): Request

A shorthand for setting the body as JSON string, so you don't have to call JSON.stringify yourself.

• data: an object or an array of objects

---

setInterceptionAllowed(allowed: boolean): Request

Sets interception flag setting for request. True by default

• allowed: a boolean value indicating whether interception is allowed or not

---

setAbortController(abortController: AbortController): Request

Sets the AbortController for the request so you can manually abort it.

• abortController: an AbortController instance

---

setUrlParam(key: string, value: string | number): Request

Sets a URL parameter. It will replace the occurrence of :key in the URL path.

• key: parameter key
• value: parameter value

/users/:id -> setUrlParam("id", 2) -> /users/2

---

setSearchParam(key: string, value: string | number | boolean | Array<string | number | boolean>): Request

Sets a query parameter. It will append the key-value pair to the URL.

• key: query parameter key
• value: query parameter value

setSearchParam("name", "John") -> /users?name=John
setSearchParam("names", ["John", "Alice"]) -> /users?names[]=John&names[]=Alice

---

setSearchParams(params: Record<string, string | number | boolean | Array<string | number | boolean>>): Request

Sets multiple query parameters. It will append the key-value pairs to the URL.

• params: an object with key-value pairs representing the query parameters

setSearchParams({ name: "John", age: 30 }) -> /users?name=John&age=30
setSearchParams({ names: ["John", "Alice"] }) -> /users?names[]=John&names[]=Alice

---

Subclasses of Request

• Get
• Post
• Put
• Delete
• Patch
• Head

These subclasses are convenience classes that extend Request and set the method property accordingly.

---

Client Class

Properties

• baseURL: string
• defaultHeaders: Record<string, string>

Constructor

constructor(baseURL: string)

Creates a new Client instance with a base URL.

• baseUrl: the base URL for the client

Methods

setTransport(transport: Transport<Response>): Client<Response>

Sets the transport for the client.

• transport: a Transport object implementation

---

setDefaultHeaders(headers: Record<string, string>): Client<Response>

Sets the default headers for the client.

• headers: an object with key-value pairs representing the default headers

---

setInterceptor(interceptor: Interceptor<Response>): Client<Response>

Sets the interceptor for the client.

• interceptor: an Interceptor object implementation

---

perform(request: Request): Promise<Response>

Performs the given request and returns a response Promise.

• request: any Request instance including Post, Get etc.

---

Transport<T> Interface

interface Transport<T> {
  handle(request: Request): Promise<T>;
}

The Transport interface defines a single method handle that takes a Request instance and returns a Promise that resolves with the response of T type.

Interceptor<T> Interface

interface Interceptor<T> {
  onResponse(request: Request, response: T, promise: OriginalPromise<T>): Promise<void>;
}

The Interceptor<T> interface defines a single method onResponse that is called with the request, response, and the original Promise. It can be used to modify the response or handle errors.

• request: Request – original request object
• response: T – received response
• promise: OriginalPromise<T> - original Promise handles (resolve and reject functions)

type OriginalPromise<T> = {
  resolve: (value: T) => void;
  reject: (reason?: unknown) => void;
};