Promise Throttle  

This is a small (~500B minified and compressed) dependency-free library to limit the amount of promises run per unit of time. It is useful for scenarios such as Rest APIs consumption, where we are normally rate-limited to a certain amount of requests per time.

It doesn't have any dependencies. If you are running this on Node.js, you will need to pass whatever Promise library you are using in the constructor.

Then, you add functions to the PromiseThrottle that, once called, return a Promise.

Use

The library can be used either server-side or in the browser.

  var PromiseThrottle = require('promise-throttle');
  /**
   * A function that once called returns a promise
   * @return Promise
   */
  var myFunction = function(i) {
    return new Promise(function(resolve, reject) {
      // here we simulate that the promise runs some code
      // asynchronously
      setTimeout(function() {
        console.log(i + ": " + Math.random());
        resolve(i);
      }, 10);
    });
  };

  var promiseThrottle = new PromiseThrottle({
    requestsPerSecond: 1,           // up to 1 request per second
    promiseImplementation: Promise  // the Promise library you are using
  });

  var amountOfPromises = 10;
  while (amountOfPromises-- > 0) {
    promiseThrottle.add(myFunction.bind(this, amountOfPromises))
      .then(function(i) {
        console.log("Promise " + i + " done");
      });
  }

  // example using Promise.all
  var one = promiseThrottle.add(myFunction.bind(this, 1));
  var two = promiseThrottle.add(myFunction.bind(this, 2));
  var three = promiseThrottle.add(myFunction.bind(this, 3));

  Promise.all([one, two, three])
    .then(function(r) {
        console.log("Promises " + r.join(", ") + " done");
    });

Options

weight

You can specify weight option for each promise to dynamically adjust throttling depending on action "heaviness". For example, action with weight = 2 will be throttled as two regular actions. By default weight of all actions is 1.

  var regularAction = promiseThrottle.add(performRegularCall());
  var heavyAction = promiseThrottle.add(performHeavyCall(), {weight: 2});

signal

You can cancel queued promises using an AbortSignal. For this, pass a signal option obtained from an AbortController. Once it is aborted, the promises queued using the signal will be rejected.

If the environment where you are running the code doesn't support AbortController, you can use a polyfill.

  var controller = new AbortController();
  var signal = controller.signal;
  var pt = createPromiseThrottle(10);
  pt.addAll([
    function() {
      return fetch('example.com/a');
    },
    function() {
      return fetch('example.com/b');
    },
    function() {
      ...
    }
  ], {signal: signal});
  ...

  // let's abort the promises
  controller.abort();

You can decide to make only specific promises abortable:

  var controller = new AbortController();
  var signal = controller.signal;
  var pt = createPromiseThrottle(10);
  pt.add(function() { return fetch('example.com/a') });
  pt.add(function() { return fetch('example.com/b') }, {signal: signal});
  pt.add(function() { return fetch('example.com/c') });
  ...

  // let's abort the second one
  controller.abort();

When aborting, the promise returned by add or addAll is rejected with a specific error:

  var controller = new AbortController();
  var signal = controller.signal;
  var pt = createPromiseThrottle(10);
  pt.addAll([
    function() {
      return fetch('example.com/a');
    },
    function() {
      return fetch('example.com/b');
    },
    function() {
      ...
    }
  ], {signal: signal}).catch(function(e) {
    if (e.name === 'AbortError') {
      console.log('Promises aborted');
    }
  });
  ...

  // let's abort the promises
  controller.abort();

Installation

For node.js, install the module with: npm i promise-throttle

If you are using it in a browser, you can use bower: bower install promise-throttle

Development

Install the dependencies using npm install. Run npm start to lint, test and browserify promise-thottle.

Projects using it

See how some projects are using it:

• ivasilov/promised-twitter
• JMPerez/spotify-dedup
• johannesss/randify
• JoseBarrios/mturk-api
• zackiles/lucy-bot

License

MIT