SynchronousAsync.js

Infrastructure.js

let cache = new Map();
let pending = new Map();

function fetchTextSync(url) {
  if (cache.has(url)) {
    return cache.get(url);
  }
  if (pending.has(url)) {
    throw pending.get(url);
  }
  let promise = fetch(url).then(
    response => response.text()
  ).then(
    text => {
      pending.delete(url);
      cache.set(url, text);
    }
  );
  pending.set(url, promise);
  throw promise;
}

async function runPureTask(task) {
  for (;;) {
    try {
      return task();
    } catch (x) {
      if (x instanceof Promise) {
        await x;
      } else {
        throw x;
      }
    }
  }
}

Program.js

function getUserName(id) {
  var user = JSON.parse(fetchTextSync('/users/' + id));
  return user.name;
}

function getGreeting(name) {
  if (name === 'Seb') {
    return 'Hey';
  }
  return fetchTextSync('/greeting');
}

function getMessage() {
  let name = getUserName(123);
  return getGreeting(name) + ', ' + name + '!'; 
}

runPureTask(getMessage).then(message => console.log(message));

This is awesome. Couple of questions:

• Why does runPureTask() on line 18 have .then after it? Isn't the point to be able to call it all synchronously?
• What are perf implications in major browsers on relying on throwing the exception?

@jaredpalmer The point is not for the whole program and event loop to be synchronous. Just a small part of it. At some point you have to have a coordinator that says what happens to the rest of the program while you're blocked.

Note that this is not throwing an Error. Creating an error object can some times be a bit more expensive since that stores the stack trace. Throwing is not necessarily zero cost, but it's minor compared to the I/O that is happening here.

This thing is very much optimizing for most things to not block on I/O and is cached. The major downside is not the throw, it is that you may have to reuse some work if it does. If you're expecting to mostly hit I/O in many places I'd recommend async/await instead.

@jaredpalmer @sebmarkbage

I added promise rejection scenario (see "try this" comment) and composition of fetchTextSync:

https://gist.github.com/idibidiart/69fd5f9df339b5ef1783e6a8fae9fa51

I like how I can compose idiomatically with this, e.g.: let result = fetchSync('/test/1a') + ' ' + fetchSync('/test/1b')