promise-singleflight

A simple utility to ensure that concurrent calls with the same key share the same Promise, preventing duplicate executions.

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Features

• 🌀 Prevents duplicate async calls for the same key
• 🪝 TypeScript support
• 🛠️ Simple API, easy integration

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Installation

npm install --save promise-singleflight

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Usage

Basic Example

const PromiseSingleflight = require("promise-singleflight");
const singleflight = PromiseSingleflight.create();

function delay(ms) {
  return new Promise(resolve => setTimeout(resolve, ms));
}

async function queryData(path) {
  return singleflight(path, async () => {
    console.log(`start queryData ${path}`);
    await delay(1000);
    console.log(`end queryData ${path}`);
    return `data:${path}`;
  });
}

(async function () {
  const items = await Promise.all([
    queryData('/a'),
    queryData('/a'),
    queryData('/b'),
  ]);
  console.log(items.join('\n'));
})();

Output:

start queryData /a
start queryData /b
end queryData /a
end queryData /b
data:/a
data:/a
data:/b

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API

create<Key, ReturnValue>(): (key: Key, fn: () => Promise<ReturnValue>) => Promise<ReturnValue>

Creates a singleflight wrapper function.

• key: Any value that can be used as a Map key (string, number, etc.)
• fn: An async function returning a Promise

Returns: A Promise that resolves to the result of fn. If multiple calls are made with the same key before the first resolves, they will all share the same Promise.

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TypeScript Example

import { create } from "promise-singleflight";

const singleflight = create<string, number>();

async function fetchNumber(key: string): Promise<number> {
  return singleflight(key, async () => {
    // ...fetch or compute
    return 42;
  });
}

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Why use promise-singleflight?

• Prevents redundant network/database calls
• Reduces server load and improves efficiency
• Ensures data consistency for concurrent requests

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Reference

• TypeScript 4.5: New File Extensions
• Example TypeScript Package
• ts-jest
• ts-node
• eslint-config-standard-with-typescript

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License

MIT