waitgroup.js

function newWaitGroup(state = 0) {
	const cbs = []
	const wg = {}
	function invoke() {
		let cb
		while ((cb = cbs.shift())) {
			if (typeof cb != "function") {
				throw new TypeError(
					"A callback function should be passed to WaitGroup.wait()"
				)
			}
			cb()
		}
	}
	wg.add = function add(delta = 1) {
		if (state + delta < 0) {
			throw new RangeError()
		}

		state += delta
		if (state == 0) {
			invoke()
		}

		return state
	}
	wg.done = function done() {
		wg.add(-1)
	}
	wg.wait = function wait(callback) {
		cbs.push(callback)
	}

	return wg
}