How we use Async Iterators to simplify streaming data

20 May 2019

Jamie McCrindle

Head of Engineering

Async generators and async iterators

We're using React and TypeScript on the front-end in the Banking for Business team in Investec UK. We've been striving to keep our code as simple as possible which motivated our choice of MobX on the front-end over Redux. We do occasionally need to build functionality that needs to react to a stream of events and we've been discussing bringing RxJS into our stack.
In keeping with our aim to keep the code simple, we decided to see if Async Iterators could solve the same problems that RxJS would in way that is easier to reason about. This blog provides an overview of what Async Iterators are and how they compare to RxJS both in terms of usage and performance.

Async Generators

Async Generators make it easier to write code to create and consume asynchronous streams of data in a familiar imperative way. The simplest way to create an async iterator is to use async function*. In the rangeWithDelay example below we return the whole numbers between start and end with a delay of waitForMilliseconds

async function* rangeWithDelay(start, end, waitForMilliseconds) { for (let i = start; i < end; i++) { yield i; await delay(waitForMilliseconds); } }
You can go through the items return by an async iterator with for await e.g.:

for await(const item of rangeWithDelay(1, 10, 1000)) { console.log(item); }
One of the services we offer our customers is a notification when an exchange rate between two currencies hits a certain level. Say you wanted to write a simple version of this yourself where you have your code notify you if the pound (GBP) ever reached parity with the dollar (USD). We could use to get the latest rate. To avoid overloading their servers, we only want to call their API once a day.

// get the GBP / USD rate once a day async function* liveRates() { while(true) { // fetch the rates const response = await fetch(''); // parse the json body const json = await response.json(); // return the current GBPUSD exchange rate yield json.rates.USD; // pause for a day await delay(24 * 60 * 60 * 1000); } } // go through the prices as they come in for await(const price of liveRates()) { // insert brexit joke here if(price <= 1) { console.log('yikes'); break; } } // a function that pauses for a set amount of time function delay(timeout) { return new Promise(resolve => setTimeout(resolve, timeout)); }

Async Iterators

async function* is an async generator. The mechanism they use internally is the AsyncIterator interface. Here is how we could write the interval method from RxJS using AsyncIterator:

function interval(milliseconds) { let running = true; let i = 0; return { [Symbol.asyncIterator]() { return this; }, next(value) { return running ? delay(milliseconds) .then(() => ({ value: i++, done: false })) : Promise.resolve({ done: true }); }, throw(error) { running = false; return Promise.resolve({ done: true }); }, return(value) { running = false; return Promise.resolve({ done: true }); } }; }
As long as a JavaScript object has the following properties it can be used as an AsyncIterator:
  • A method called: [Symbol.asyncIterator]()
  • [Symbol.asyncIterator]() returns an object with a next() method
  • next() returns a Promise
  • If this is the last value, the Promise should resolve to { done: true }
  • Otherwise it should resolve to { value: value, done: false } where value is the next value
This is somewhat more complicated than the async function* syntax but can be used to solve problems that async function* can't. For example, it is useful for turning a stream of events into an async iterable.

Turning a stream of events into an async iterable

Async iterators make it possible to turn code that requires callbacks to one that has control flow that's easier to follow. At Investec, we often have to take streams of data from various input sources, tranform it and send it on to various output sources. The standard mechanism to support reading streams of data is event based, for example to read lines from a file the event based syntax looks something like this:

lineReader.on("line", line => { console.log(line); }); lineReader.on("close", () => { console.log("done"); });
Instead, it's easier to reason about code that is more imperative e.g. using async iterators we could imagine having code like this instead:

for await (const line of fromLineReader(lineReader)) { console.log(line); } console.log('done');
It turns out that doing this generically is somewhat complicated because:
  • lineReader could send lines before the for await consumes them
  • lineReader may need to pause sending lines so that we don't run out of memory
  • for await could run before there are any lines available
  • the async iterator next method could be called directly multiple times without waiting for new lines
The Axax library has an implementation of Subject for async iterators which borrows the concept of a Subject from RxJS. It opens up the possibility of writing something like this:

function fromLineReader(lineReader) { const subject = new Subject(); // send a line to the subject lineReader.on('line' => subject.onNext(line)); // close the subject when the line reader closes lineReader.on('close' => subject.onCompleted()); // close the line reader when the subject closes subject.finally(() => lineReader.close()); // return the async iterator return subject.iterable; }
Note: this does not handle backpressure i.e. if the lineReader is producing lines faster than the async iterator is being consumed, this process could run out of memory.

Cancellation and avoiding leaks

Neither Promises nor async iterators have a reliable way to cancel them. There is a TC39 proposal for cancellation of asynchronous operations but it is still at stage 1.
In the example below, we create an async iterable that gets stuck waiting for an upstream async iterable that never ends. Even though we call return() on it, there is no way for the neverEnds() to get cancelled and as a result the finallyblock is never called.

async function* asyncIterable() { try { // never ends is a async iterator that never // produces a value or ends for await (const item of neverEnds()) { yield item; } } finally { // never called console.log("clean up resources"); } } const iter = asyncIterable(); await iter.return();

Async Iterators vs RxJS

Reactive Extensions for JavaScript or RxJS is another way to manage asynchronous streams. A significant difference between the two is in how control flows when using them. For example this is the control flow around an async iterable for await:

console.log('starting'); try { for await (const item of source) { console.log('in the loop'); } console.log('succeeded'); } catch(error) { console.log('error'); } finally { console.log('done'); } console.log('on to the next thing');
While the for await syntax is new, the rest of the control flow behaves as you'd expect, especially for the try / catch / finally blocks
And this is the equivalent control flow using RxJS

While the for await syntax is new, the rest of the control flow behaves as you'd expect, especially for the try / catch / finally blocks
And this is the equivalent control flow using RxJS

console.log("starting"); const subscribe = source .pipe( finalize(() => { console.log("done"); console.log("on to the next thing"); }) ) .subscribe( item => console.log("in the loop"), error => console.log("error"), () => console.log("succeeded") );
While this will be familiar to RxJS developers it may be harder for developers unfamiliar with the library to reason about.
The other significant difference is in how they perform.

Performance vs RxJS

We compared the performance of async iterators to RxJS. As with all benchmarks, these should only be considered directional. Async iterators are still relatively new and performance will vary across javascript engine, operating system, hardware and the problem you're trying to solve.
We used the benchmark.js library in all of these benchmarks.
Note: the implementations are not exactly equivalent but match how these problems would be idiomatically solved using each framework.

Test system setup

Property Value
Node 10.9.0
Hardware 2.7 GHz Intel Core i7
Operating System Mac OS X

A note about RxJS Schedulers

Performance of RxJS depends a lot on which scheduler you use. Async Iterators are comperable to the asapScheduler.
null By not passing any scheduler, notifications are delivered synchronously and recursively. Use this for constant-time operations or tail recursive operations.
queueScheduler Schedules on a queue in the current event frame (trampoline scheduler). Use this for iteration operations.
asapScheduler Schedules on the micro task queue, which is the same queue used for promises. Basically after the current job, but before the next job. Use this for asynchronous conversions.
asyncScheduler Schedules work with setInterval. Use this for time-based operations.
animationFrameScheduler Schedules task that will happen just before next browser content repaint. Can be used to create smooth browser animations.


In this benchmark we implemented sum as a reduce over 1000 numbers.

async function sum(source) { let accumulator = 0; for await (const item of source) { accumulator += item; } return accumulator; } await sum(source);
Rx.from(source) .pipe(RxOperators.reduce((a, n) => a + n, 0)) .subscribe(/* do something with the result */);


Implementation Ops Per Second*
RxJS default 15,912.45 per second
RxJS queued 5,353.40 per second
RxJS asap 1,022.07 per second
AsyncIterators 1,815.12 per second
* Higher is better

map filter reduce

In this case we combine a map, filter and a reduce over 1000 numbers.

async function mapFilterReduce(source) { let accumulator = 0; for await (const x of Native.from(array)) { if (x % 2 === 0) { accumulator = accumulator + x * 2; } } } await mapFilterReduce(source);
Rx.from(array, scheduler) .pipe( RxOperators.filter(x => x % 2 === 0), => x * 2), RxOperators.reduce((a, n) => a + n, 0) ) .subscribe(/* do something with the result */);


Implementation Ops Per Second*  
RxJS default 10,599.15 per second  
RxJS queued 2,711.63 per second  
RxJS asap 801.94 per second  
AsyncIterators 1,781.83 per second  
async function* from(array) { for await (const item of array) yield item; } async function concatMapReduce(source) { let accumulator = 0; for await (const outer of source) { for await (const inner of from(outer)) { accumulator = accumulator + inner; } } } await concatMapReduce(source);
* Higher is better

concat map reduce

In this case the source is an array of arrays of numbers that is 100x100.

async function* from(array) { for await (const item of array) yield item; } async function concatMapReduce(source) { let accumulator = 0; for await (const outer of source) { for await (const inner of from(outer)) { accumulator = accumulator + inner; } } } await concatMapReduce(source);
Rx.from(array, scheduler) .pipe( RxOperators.concatMap(value => Rx.from(value)), RxOperators.reduce((a, n) => a + n, 0) ) .subscribe(/* do something with the result */);


Implementation Ops Per Second*
RxJS default 2,889.56 per second
RxJS queued 1,845.89 per second
RxJS asap 1,242.96 per second
AsyncIterators 182.97 per second
* Higher is better

vs Most

We compared async iterators to RxJS because it's likely that folk will be choosing between these two ways of solving problems involving async streams given RxJS's popularity and that async iterators are now part of ECMAScript.
If you're very performance sensitive, you may consider other libraries e.g. most but it will depend on the problem you're solving. Here are the same benchmarks including most.


Variation Implementation Ops Per Second*
reduce RxJS 15,912.45 per second
reduce AsyncIterators 1,815.12 per second
reduce Most 66,560.13 per second
mapFilterReduce RxJS 10,599.15 per second
mapFilterReduce AsyncIterators 1,781.83 per second
mapFilterReduce Most 34,252.84 per second
concatMapReduce RxJS 2,889.56 per second
concatMapReduce AsyncIterators 182.97 per second
concatMapReduce Most 2,318.10 per second
* Higher is better

Native vs Transpiled

It's likely that you'll want to transpile your async iterators to support all browsers.

Browser Support

Browser Supported
IE 11 No
Edge No
Firefox Yes
Chrome Yes
Safari 12 Yes
We tested the performance of the reduce implementation above using Babel and TypeScript to transpile the code.


Variation Implementation Ops Per Second*
reduce TypeScript 3.0.2 8,967.30 per second
reduce Babel 6.26.0 15,101.20 per second
reduce Native 19,129.27 per second
* Higher is better
Interestingly Typescript transpilation results in a 50% slowdown while Babel is quite close to native performance. Babel requires the inclusion of the regenerator-runtime from Facebook.

So did we end up using Async Iterators

Yes! We have used them to build our internal dashboard application. It's built using React, TypeScript, Koa, SocketIO and Immer. Monitors run on the server and publish metric deltas to client browsers to update their status but this is perhaps a topic for another blog entry. It turns out that Async Generators are a great way to write our monitors in a simple readable way.