Add idle event

[Ilaiwi]

following #107

I added an idle event

idle

Emitted when the queue becomes empty, and all promises have completed; queue.size === 0 && queue.pending === 0.

const delay = require('delay');
const {default: PQueue} = require('p-queue');

const queue = new PQueue();

queue.on('idle', () => {
	console.log(`Queue is idle.  Size: ${queue.size}  Pending: ${queue.pending}`);
});

const job1 = queue.add(() => delay(2000));
const job2 = queue.add(() => delay(500));

await job1;
await job2;
// => 'Queue is idle.  Size: 0  Pending: 0'

await queue.add(() => delay(600));
// => 'Queue is idle.  Size: 0  Pending: 0'

[Ilaiwi]

@sindresorhus PR updated

[rightaway]

What's the difference between onIdle() and the idle event? There's also onEmpty which doesn't have an event equivalent. When to use the function vs event?

[sindresorhus]

onIdle resolves only once after being called, while idle is called every time the queue is idle.