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https://github.com/tootallnate/once
Creates a Promise that waits for a single event
https://github.com/tootallnate/once
Last synced: 1 day ago
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Creates a Promise that waits for a single event
- Host: GitHub
- URL: https://github.com/tootallnate/once
- Owner: TooTallNate
- License: mit
- Created: 2020-02-08T23:44:58.000Z (almost 5 years ago)
- Default Branch: master
- Last Pushed: 2021-09-27T09:13:30.000Z (about 3 years ago)
- Last Synced: 2024-12-17T18:10:22.596Z (9 days ago)
- Language: TypeScript
- Size: 17.6 KB
- Stars: 24
- Watchers: 3
- Forks: 7
- Open Issues: 0
-
Metadata Files:
- Readme: README.md
- License: LICENSE
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README
# @tootallnate/once
### Creates a Promise that waits for a single event
## Installation
Install with `npm`:
```bash
$ npm install @tootallnate/once
```
## API
### once(emitter: EventEmitter, name: string, opts?: OnceOptions): Promise<[...Args]>
Creates a Promise that waits for event `name` to occur on `emitter`, and resolves
the promise with an array of the values provided to the event handler. If an
`error` event occurs before the event specified by `name`, then the Promise is
rejected with the error argument.
```typescript
import once from '@tootallnate/once';
import { EventEmitter } from 'events';
const emitter = new EventEmitter();
setTimeout(() => {
emitter.emit('foo', 'bar');
}, 100);
const [result] = await once(emitter, 'foo');
console.log({ result });
// { result: 'bar' }
```
#### Promise Strong Typing
The main feature that this module provides over other "once" implementations is that
the Promise that is returned is _**strongly typed**_ based on the type of `emitter`
and the `name` of the event. Some examples are shown below.
_The process "exit" event contains a single number for exit code:_
```typescript
const [code] = await once(process, 'exit');
// ^ number
```
_A child process "exit" event contains either an exit code or a signal:_
```typescript
const child = spawn('echo', []);
const [code, signal] = await once(child, 'exit');
// ^ number | null
// ^ string | null
```
_A forked child process "message" event is type `any`, so you can cast the Promise directly:_
```typescript
const child = fork('file.js');
// With `await`
const [message, _]: [WorkerPayload, unknown] = await once(child, 'message');
// With Promise
const messagePromise: Promise<[WorkerPayload, unknown]> = once(child, 'message');
// Better yet would be to leave it as `any`, and validate the payload
// at runtime with i.e. `ajv` + `json-schema-to-typescript`
```
_If the TypeScript definition does not contain an overload for the specified event name, then the Promise will have type `unknown[]` and your code will need to narrow the result manually:_
```typescript
interface CustomEmitter extends EventEmitter {
on(name: 'foo', listener: (a: string, b: number) => void): this;
}
const emitter: CustomEmitter = new EventEmitter();
// "foo" event is a defined overload, so it's properly typed
const fooPromise = once(emitter, 'foo');
// ^ Promise<[a: string, b: number]>
// "bar" event in not a defined overload, so it gets `unknown[]`
const barPromise = once(emitter, 'bar');
// ^ Promise
```
### OnceOptions
- `signal` - `AbortSignal` instance to unbind event handlers before the Promise has been fulfilled.