# node-singleflight

Suppress duplicate function calls with the same key while one call is still in flight. Duplicate callers share the same result or rejection.

## Install

```sh
npm install node-singleflight
```

```sh
yarn add node-singleflight
```

## Usage

JavaScript using the existing CommonJS API:

```js
const singleflight = require('node-singleflight')

async function example() {
  return singleflight.Do('SomeKey', async () => {
    const data = await doSomething()
    return processData(data)
  })
}
```

The equivalent TypeScript version uses the declarations included with the package:

```ts
import { Do } from 'node-singleflight'

async function example(): Promise<ProcessedData> {
  return Do('SomeKey', async () => {
    const data = await doSomething()
    return processData(data)
  })
}
```

## API

### `Do(key, func)`

Runs `func` once while the given key is in flight and returns a Promise for its result.

- Calls with the same key share the same result or rejection.
- Calls with different keys run independently.
- Keys use JavaScript `Map` identity semantics; object keys match by reference.
- The key is removed after fulfillment or rejection, so a later call can retry.
- `func` may return a value, a Promise, or any Promise-like value.

`func` must eventually settle. If it never fulfills or rejects, calls using the same key will continue waiting. Apply a timeout or cancellation inside `func` when the underlying operation can hang.