Handle costly promise in NodeJS

costly.js

// Timeout promise for delaying something
function timeout(millis) {
	return new Promise((resolve, reject) => {
		setTimeout(() => {
			resolve();
		}, millis);
	})
}

// Costly function, it require 1sec every invocation.
let counter = 0;
function doSomethingCostly(msg) {
	console.log(`doSomethingCostly('${msg}')...`);
	let count = ++counter;
	return timeout(1000).then(() => `${msg} - ivocation #${count}`);
}

// This method ensure to manage costly functiom
let running = false;
let subscriptions = [];
function doSomething(msg) {
	console.log(`doSomething('${msg}')...`)
	return new Promise((resolve, reject) => {
		// Use subscription style, register callback function
		let f = (out) => {
			resolve(out);
		};
		subscriptions.push(f);

		if (!running) {
			// The actual work
			running = true;
			doSomethingCostly(msg).then((out) => {
				running = false;

				let subs = subscriptions
				subscriptions = []

				subs.forEach((f) => f(out));
			});
		}
	});
}


// foo and bar will share single costly function invocation
doSomething('foo').then((out) => {
	console.log(out);
});

doSomething('bar').then((out) => {
	console.log(out);
});


// baz will use new invocation
timeout(1020).then(() => {
	doSomething('baz').then((out) => {
		console.log(out);
	});
})