Rate limiting for Meteor in 2021

rate-limit.js

/*
Ensures that a function "fn" is only run every "delay" milliseconds.

Let's say you have two meteor users, and they want to call a function repeatedly to send 300 emails each.
Your mailing API blocks you if you go over 2 req per second.

Your SERVER code would look like this

// the expensive function
function rawSendMail(a,b,c){
  // (this==null here)
  return callSomeExpensiveFunctionThatReturnsResult()
}

// the rate limited version, will not run more than twice a second
const sendMail = rateLimit(rawSendMail, 550) ;

// just call it synchronously in meteor
Meteor.methods({
  mailBatch(listOfEmails){
    return listOfEmails.map(sendMail)
  }
})



*/
export function rateLimit(fn, delay) {
  var queue = [],
    lastRun = 0,
    running = false;

  function delayTillNext() {
    return Math.max(0, delay - (Date.now() - lastRun));
  }
  function processQueue() {
    lastRun = Date.now();
    var item = queue.shift();
    if (item) {
      try {
        item.resolve(fn.apply(null, item.arguments));
      } catch (e) {
        item.reject(e);
      }
    }
    if (queue.length === 0) {
      running = false;
    } else {
      running = true;
      Meteor.setTimeout(processQueue, delayTillNext());
    }
  }

  return function limited() {
    const item = {
      arguments: [].slice.call(arguments)
    };
    const promise = new Promise((resolve, reject) => {
      item.resolve = resolve;
      item.reject = reject;
    });
    queue.push(item);
    if (!running) {
      running = true;
      Meteor.setTimeout(processQueue, delayTillNext());
    }
    return promiseToSync(promise);
  };
}

// Little helper to make server code wait for a Promise, there's surely a better way
function promiseToSync(promise) {
  return Meteor.wrapAsync(cb =>
    promise.then(
      res => cb(null, res),
      err => cb(err)
    )
  )();
}