Category: type Since: 1.0.0 Tags: type, function, async, promise, generic

AsyncFn

Represents an asynchronous function.

Usage

import type { AsyncFn } from "@petr-ptacek/js-lab";

const fetchUser: AsyncFn<[id: string], User> = async (id) => {
  const response = await fetch(`/api/users/${id}`);
  return response.json();
};

const saveData: AsyncFn<[data: object, filename: string], void> = async (data, filename) => {
  await fs.writeFile(filename, JSON.stringify(data));
};

// Retry utility
async function withRetry<TArgs extends unknown[], TResult>(
  fn: AsyncFn<TArgs, TResult>,
  args: TArgs,
  maxAttempts = 3
): Promise<TResult> {
  for (let attempt = 1; attempt <= maxAttempts; attempt++) {
    try {
      return await fn(...args);
    } catch (error) {
      if (attempt === maxAttempts) throw error;
      await new Promise((resolve) => setTimeout(resolve, 1000 * attempt));
    }
  }
  throw new Error("Max attempts reached");
}

Why This Type Exists

Asynchronous operations are fundamental in modern JavaScript applications. AsyncFn provides a standardized, generic type for async functions with proper argument and return type constraints, enabling type-safe composition, utilities, and higher-order async function patterns.

Type Declaration

type AsyncFn<TArgs extends unknown[] = unknown[], TResult = unknown> = (...args: TArgs) => Promise<TResult>;

Type Parameters

• <TArgs>: Tuple type representing the function's parameter types (defaults to unknown[]).
• <TResult>: The type of value the Promise resolves to (defaults to unknown).

When To Use

Use AsyncFn when building generic async utilities, middleware systems, or APIs that work with various async function signatures while maintaining type safety.

Summary

AsyncFn provides a standardized type for asynchronous functions with flexible parameter and return type constraints, enabling type-safe async utilities and composition patterns.