PromiEvent

A TypeScript implementation of a Promise that can also emit events.

A PromiEvent is a JavaScript Promise that is also a Node.js EventEmitter. This allows a Promise to emit lifecycle events before a Promise is resolved or rejected.

Installing

Using node.js:

npm install --save promievent

Node version support

Version 0.1.x supports PromiEvent.finally() so needs to run on Node.js 10 or higher as it uses native Promise.prototype.finally. See Node.js ES2015 Support for more details.

Version 0.0.x works on Node.js 8.

Usage

A PromiEvent can be used as a standard promise or an EventEmmiter, but it's best when used together. In the following example, the promise is resolved after 10 seconds but on each second a interval event is emitted. An event is also emitted on the first and last second.

const promiEvent = new PromiEvent<number>((resolve, reject) =>
{
    let counter = 0

    const timer = setInterval(()=>
    {
        counter++
        
        promiEvent.emit('interval', counter)

        if (counter === 1) {
            promiEvent.emit('first', counter)
        }

        if (counter === 10) {
            promiEvent.emit('last', counter)
            resolve(counter)

            // stop the timer
            clearInterval(timer)
        }
    }, 100)
})

The following registers an interval event listener with the EventEmitter. This will print 10 lines.

promiEvent.on('interval', (count: number)=>
{
    console.log(`interval number ${count}`)
})

The following register an interval event listener that is only fired once. This will only print one line.

promiEvent.once('interval', (count: number)=>
{
    console.log(`interval number ${count}`)
})

The following will resolve the promise and print one line with a count of 10.

promiEvent.then((count)=>
{
    console.log(`interval number ${count}`)
}).catch((err)=>
{
	console.log(`Something bad happened ${err.message}`)
	// error handling
})

Or you can resolve the promise using async/await assuming this code is in an async function.

try {
	const count = await promiEvent
	console.log(`interval number ${count}`)
}
catch (err) {
	console.log(`Something bad happened ${err.message}`)
	// error handling
}

Credits

The inspiration for a Promise that emits events has come from Fabian Vogelsteller's Callbacks Promises Events in the Ethereum web3.js 1.x project. The JavaScript implementation is in web3-core-promievent.

The TypeScript implementation also got influence from Matthew Mueller's comments on an issue in Microsoft's TypeScript GitHub repo.

License

The MIT Licence.

See LICENSE.