@prexjs/promise

Lightweight, zero-dependency type guard utility to test if a value is a Promise or Thenable.

@prexjs/promise provides an ultra-fast, robust TypeScript type guard isPromise(val) to determine if a value is an ES6 Promise, async thenable, or third-party promise (Bluebird, Q, etc.).

Features

• Validates whether an object or function qualifies as a Promise/A+ thenable.
• TypeScript user-defined type guard narrowing (value is PromiseLike<T>).
• Dual ES Module (.mjs) and CommonJS (.cjs) support with TypeScript definitions.
• Zero runtime dependencies.

Installation

Install using your preferred package manager:

# npm
npm install @prexjs/promise

# pnpm
pnpm add @prexjs/promise

# Yarn
yarn add @prexjs/promise

# Bun
bun add @prexjs/promise

Quick start

import isPromise from '@prexjs/promise';

console.log(isPromise(Promise.resolve('antigravity'))); // true
console.log(isPromise({ then: (resolve) => resolve(1) })); // true
console.log(isPromise('not a promise')); // false
console.log(isPromise(null)); // false

Both default and named exports are supported:

import isPromise, { isPromise as namedIsPromise } from '@prexjs/promise';

Examples

Handling synchronous or asynchronous results

import isPromise from '@prexjs/promise';

function handle(result: unknown) {
  if (isPromise(result)) {
    result.then((val) => {
      console.log('Async resolved:', val);
    });
  } else {
    console.log('Sync result:', result);
  }
}

Compatibility

@prexjs/promise runs anywhere ECMAScript executes (Node.js, Deno, Bun, and all browsers).

License

This project is licensed under the MIT License. See LICENSE for the full license text.

Related projects

• prex.js — prex.js monorepo and ecosystem.