Type safe API Caller

A minimalistic TypeScript/JavaScript library for making type-safe API calls using a functional approach. Every API call returns either an APISuccess or APIFailure result - no exceptions to catch, just two clear outcomes to handle.

Features

• Type-safe responses - Discriminated union pattern ensures compile-time safety
• No exception handling - API calls never throw; always return success or failure
• Retry support - Built-in retry mechanism with configurable intervals and error filtering
• Hook system - Register callbacks for request start, end, and retry events
• Response time tracking - Every response includes execution time in milliseconds
• Custom fetch support - Override the native fetch with your own implementation

Installation

npm install typesafe-api-call

Quick Start

import { APICaller, APIResponse, type APIRequest, APISuccess } from 'typesafe-api-call';

async function getUser(id: number): Promise<APIResponse<User, unknown>> {
  const apiRequest: APIRequest = {
    url: new URL(`https://api.example.com/users/${id}`),
    method: 'GET'
  };

  return APICaller.call(apiRequest, (response: unknown) => {
    // Return decoded User or null if decoding fails
    return decodeUser(response);
  });
}

const result = await getUser(1);

if (result instanceof APISuccess) {
  console.log(result.response); // Type-safe User object
} else {
  console.log(result.errorMessage); // Handle failure
}

API Reference

APICaller.call()

Makes a single API call and returns a typed response.

APICaller.call<SuccessResponse, ErrorResponse>(
  apiRequest: APIRequest,
  responseDecoder: ResponseDecoder<SuccessResponse>,
  errorResponseDecoder?: ResponseDecoder<ErrorResponse> | null,
  apiCaller?: FetchType
): Promise<APIResponse<SuccessResponse, ErrorResponse>>

Parameters:

• apiRequest - Request configuration (url, method, headers, body, etc.)
• responseDecoder - Function to decode successful responses. Returns decoded value or null if decoding fails
• errorResponseDecoder - Optional function to decode error responses
• apiCaller - Optional custom fetch implementation

APICaller.callWithRetries()

Makes an API call with automatic retry on failure.

APICaller.callWithRetries<SuccessResponse, ErrorResponse>(
  apiRequest: APIRequest,
  responseDecoder: ResponseDecoder<SuccessResponse>,
  retryConfig: RetryConfig,
  errorResponseDecoder?: ResponseDecoder<ErrorResponse> | null,
  apiCaller?: FetchType
): Promise<APIResponse<SuccessResponse, ErrorResponse>>

RetryConfig:

type RetryConfig = {
  maxRetries: number;      // Number of retry attempts
  retryInterval: number;   // Delay between retries in milliseconds
  retryOn: ErrorClass[];   // Error types that trigger a retry
};

Example:

const response = await APICaller.callWithRetries(
  apiRequest,
  responseDecoder,
  {
    maxRetries: 3,
    retryInterval: 2000,
    retryOn: ['DOMException', 'TypeError', 'Error']
  }
);

APIRequest

Extends the native RequestInit interface with required fields:

interface APIRequest extends RequestInit {
  url: URL;                           // Required: Request URL
  method: HttpMethod;                 // Required: HTTP method
  agent?: unknown;                    // Optional: For node-fetch compatibility
  // ...plus all standard RequestInit options (headers, body, credentials, etc.)
}

Supported HTTP Methods: GET, HEAD, POST, PUT, DELETE, PATCH

APISuccess

Returned when the API call succeeds and response decoding is successful.

class APISuccess<T> {
  readonly statusCode: number;  // HTTP status code (e.g., 200)
  readonly status: string;      // HTTP status text (e.g., "OK")
  readonly response: T;         // Decoded response body
  readonly time: number;        // Response time in milliseconds
}

APIFailure

Returned when the API call fails or response decoding fails.

class APIFailure<E> {
  readonly errorMessage: string;       // Error description
  readonly errorCode: number;          // HTTP status code (-1 for exceptions)
  readonly response: E | null;         // Decoded error response (if decoder provided)
  readonly errorResponse: unknown;     // Raw error response
  readonly errorDetails: ErrorDetails | null;  // Detailed error info
  readonly time: number;               // Response time in milliseconds
}

ErrorDetails

Provides detailed information about errors encountered during API calls.

type ErrorDetails = {
  class: ErrorClass;          // Error classification
  name: string;               // Error name
  message: string | null;     // Error message
  cause: unknown;             // Error cause
  stack: string | null;       // Stack trace
};

type ErrorClass =
  | 'DOMException'        // Network/abort errors
  | 'TypeError'           // Invalid request/URL errors
  | 'DecodeFailure'       // Response decoding failed
  | 'InternalError'       // Internal errors
  | 'Error'               // General errors
  | 'UnhandledException'; // Unexpected errors

Hooks

Register callbacks to observe API call lifecycle events. Each hook requires a unique id to prevent duplicate registrations.

Start Hook

Triggered before an API call begins.

APICaller.registerStartHook({
  id: 'my-start-hook',
  func: (apiRequest: APIRequest) => {
    console.log(`Starting: ${apiRequest.method} ${apiRequest.url.href}`);
  }
});

End Hook

Triggered after an API call completes (success or failure).

APICaller.registerEndHook({
  id: 'my-end-hook',
  func: (apiRequest: APIRequest, response: APIResponse<unknown, unknown>) => {
    const time = response instanceof APISuccess ? response.time : response.time;
    console.log(`Completed in ${time}ms`);
  }
});

Retry Hook

Triggered when a retry attempt is made (only with callWithRetries).

APICaller.registerRetryHook({
  id: 'my-retry-hook',
  func: (apiRequest: APIRequest, response: APIResponse<unknown, unknown>, attempt: number) => {
    console.log(`Retry attempt #${attempt}`);
  }
});

Response Decoder Pattern

The ResponseDecoder function validates and transforms raw API responses into typed objects:

type ResponseDecoder<T> = (rawResponse: unknown) => T | null;

• Return the decoded value on success
• Return null if decoding/validation fails (triggers APIFailure with DecodeFailure error class)

This pattern integrates well with validation libraries like type-decoder.

Examples

GET Request

async function getAllPosts(): Promise<APIResponse<Post[], unknown>> {
  const apiRequest: APIRequest = {
    url: new URL('https://api.example.com/posts'),
    method: 'GET'
  };
  return APICaller.call(apiRequest, (response: unknown) => {
    return decodeArray(response, decodePost);
  });
}

POST Request

async function createPost(post: Post): Promise<APIResponse<CreatePostResponse, unknown>> {
  const apiRequest: APIRequest = {
    url: new URL('https://api.example.com/posts'),
    method: 'POST',
    headers: { 'Content-Type': 'application/json' },
    body: JSON.stringify(post)
  };
  return APICaller.call(apiRequest, decodeCreatePostResponse);
}

With Error Response Decoder

const response = await APICaller.call(
  apiRequest,
  decodeSuccessResponse,  // Decode success response
  decodeErrorResponse     // Decode error response if success decoding fails
);

if (response instanceof APISuccess) {
  // response.response is typed as SuccessType
} else {
  // response.response is typed as ErrorType | null
}

Handling APIFailure

When an API call fails, you receive an APIFailure instance. Here's how to interpret and handle different failure scenarios:

Understanding Failure Types

import { APICaller, APISuccess, APIFailure, type APIRequest } from 'typesafe-api-call';

const result = await APICaller.call(apiRequest, decodeUser);

if (result instanceof APIFailure) {
  // Check what type of error occurred
  const errorClass = result.errorDetails?.class;

  switch (errorClass) {
    case 'DOMException':
      // Network error, request aborted, or CORS issue
      console.log('Network error:', result.errorDetails?.message);
      break;

    case 'TypeError':
      // Invalid URL or request configuration
      console.log('Invalid request:', result.errorDetails?.message);
      break;

    case 'DecodeFailure':
      // Response received but decoder returned null
      // The raw response is available for inspection
      console.log('Failed to decode response:', result.errorResponse);
      break;

    case 'InternalError':
    case 'Error':
    case 'UnhandledException':
      // Other runtime errors
      console.log('Unexpected error:', result.errorDetails?.message);
      break;

    default:
      // HTTP error (4xx, 5xx) - errorDetails may be null
      console.log(`HTTP ${result.errorCode}:`, result.errorMessage);
  }
}

Using Typed Error Decoders

For APIs that return structured error responses, use an error decoder to get typed error handling:

// Define your API's error response structure
type APIErrorResponse = {
  code: string;
  message: string;
  details?: string[];
};

// Create a decoder for the error response
function decodeAPIError(raw: unknown): APIErrorResponse | null {
  if (
    typeof raw === 'object' &&
    raw !== null &&
    'code' in raw &&
    'message' in raw &&
    typeof (raw as APIErrorResponse).code === 'string' &&
    typeof (raw as APIErrorResponse).message === 'string'
  ) {
    return raw as APIErrorResponse;
  }
  return null;
}

// Use both success and error decoders
async function createUser(userData: UserInput): Promise<User | APIErrorResponse | null> {
  const apiRequest: APIRequest = {
    url: new URL('https://api.example.com/users'),
    method: 'POST',
    headers: { 'Content-Type': 'application/json' },
    body: JSON.stringify(userData)
  };

  const result = await APICaller.call(
    apiRequest,
    decodeUser,      // Success decoder
    decodeAPIError   // Error decoder
  );

  if (result instanceof APISuccess) {
    return result.response; // Typed as User
  }

  // Handle typed error response
  if (result.response !== null) {
    // Error was successfully decoded - result.response is typed as APIErrorResponse
    console.log(`API Error [${result.response.code}]: ${result.response.message}`);
    return result.response;
  }

  // Decoding failed or network error - fall back to raw error info
  console.log('Request failed:', result.errorMessage);
  return null;
}

Complete Error Handling Example

type ValidationError = {
  field: string;
  message: string;
};

type ServerError = {
  error: string;
  validationErrors?: ValidationError[];
  requestId?: string;
};

function decodeServerError(raw: unknown): ServerError | null {
  if (typeof raw === 'object' && raw !== null && 'error' in raw) {
    return raw as ServerError;
  }
  return null;
}

async function submitForm(formData: FormData) {
  const result = await APICaller.call(
    { url: new URL('https://api.example.com/submit'), method: 'POST', body: formData },
    decodeSuccessResponse,
    decodeServerError
  );

  if (result instanceof APISuccess) {
    console.log('Success! Completed in', result.time, 'ms');
    return { success: true, data: result.response };
  }

  // APIFailure - determine the cause
  const failure = result;

  // Case 1: Server returned a structured error response
  if (failure.response !== null) {
    const serverError = failure.response;

    if (serverError.validationErrors) {
      // Handle validation errors
      return {
        success: false,
        validationErrors: serverError.validationErrors
      };
    }

    return {
      success: false,
      error: serverError.error,
      requestId: serverError.requestId
    };
  }

  // Case 2: Network or decoding error
  if (failure.errorDetails) {
    return {
      success: false,
      error: `${failure.errorDetails.class}: ${failure.errorDetails.message}`,
      isNetworkError: ['DOMException', 'TypeError'].includes(failure.errorDetails.class)
    };
  }

  // Case 3: HTTP error without structured response
  return {
    success: false,
    error: failure.errorMessage,
    httpStatus: failure.errorCode
  };
}

APIFailure Properties Reference

Property	Type	Description
errorMessage	string	Human-readable error description
errorCode	number	HTTP status code, or -1 for exceptions
response	E | null	Decoded error response (if error decoder provided and succeeded)
errorResponse	unknown	Raw error response body
errorDetails	ErrorDetails | null	Detailed error info for exceptions
time	number	Response time in milliseconds

Checkout more examples in the examples

Development

• Clone the repository
• Install dependencies using pnpm install
• Run pnpm run build to build the library.
• You can use examples/index.ts to test your changes.

Contribution

• The library is open to all your suggestions & bugfixes and I would love to see your contributions.
• To contribute, kindly fork the repository & raise changes to the release branch.

Useful Libraries

The following libraries work well with typesafe-api-call:

• type-decoder: A library to decode and validate data types
• type-crafter: Generate types & decoders from YAML specifications

When combined, you can define types in YAML and auto-generate TypeScript types and decoders to use with typesafe-api-call.

Checkout the Example: YAML & Example: Generated Types

License

ISC