94 lines
2.6 KiB
Markdown
94 lines
2.6 KiB
Markdown
# @tootallnate/once
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### Creates a Promise that waits for a single event
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## Installation
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Install with `npm`:
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```bash
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$ npm install @tootallnate/once
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```
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## API
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### once(emitter: EventEmitter, name: string, opts?: OnceOptions): Promise<[...Args]>
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Creates a Promise that waits for event `name` to occur on `emitter`, and resolves
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the promise with an array of the values provided to the event handler. If an
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`error` event occurs before the event specified by `name`, then the Promise is
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rejected with the error argument.
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```typescript
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import once from '@tootallnate/once';
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import { EventEmitter } from 'events';
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const emitter = new EventEmitter();
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setTimeout(() => {
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emitter.emit('foo', 'bar');
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}, 100);
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const [result] = await once(emitter, 'foo');
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console.log({ result });
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// { result: 'bar' }
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```
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#### Promise Strong Typing
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The main feature that this module provides over other "once" implementations is that
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the Promise that is returned is _**strongly typed**_ based on the type of `emitter`
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and the `name` of the event. Some examples are shown below.
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_The process "exit" event contains a single number for exit code:_
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```typescript
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const [code] = await once(process, 'exit');
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// ^ number
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```
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_A child process "exit" event contains either an exit code or a signal:_
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```typescript
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const child = spawn('echo', []);
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const [code, signal] = await once(child, 'exit');
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// ^ number | null
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// ^ string | null
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```
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_A forked child process "message" event is type `any`, so you can cast the Promise directly:_
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```typescript
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const child = fork('file.js');
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// With `await`
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const [message, _]: [WorkerPayload, unknown] = await once(child, 'message');
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// With Promise
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const messagePromise: Promise<[WorkerPayload, unknown]> = once(child, 'message');
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// Better yet would be to leave it as `any`, and validate the payload
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// at runtime with i.e. `ajv` + `json-schema-to-typescript`
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```
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_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:_
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```typescript
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interface CustomEmitter extends EventEmitter {
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on(name: 'foo', listener: (a: string, b: number) => void): this;
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}
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const emitter: CustomEmitter = new EventEmitter();
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// "foo" event is a defined overload, so it's properly typed
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const fooPromise = once(emitter, 'foo');
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// ^ Promise<[a: string, b: number]>
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// "bar" event in not a defined overload, so it gets `unknown[]`
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const barPromise = once(emitter, 'bar');
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// ^ Promise<unknown[]>
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```
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### OnceOptions
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- `signal` - `AbortSignal` instance to unbind event handlers before the Promise has been fulfilled.
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