Poor man's algebraic effects in JavaScript

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));