This is a small utility to limit the execution rate of a function. It is useful for scenarios such as REST APIs consumption where the amount of requests per unit of time should not exceed a given threshold.

This module is very similar to node-function-rate-limit. The difference is that valvelet works seamlessly with promise-returning functions.

npm install --save valvelet

The module exports a single function that takes four arguments.

Returns a function which should be called instead of fn.

• fn - The function to rate limit calls to.
• limit - The maximum number of allowed calls per interval.
• interval - The timespan where limit is calculated.
• size - The maximum size of the internal queue. Defaults to 2^32 - 1 which is the maximum array size in JavaScript.

A function that returns a promise which resolves to the value returned by the original fn function. When the internal queue is at capacity the returned promise is rejected.

const valvelet = require('valvelet');

const get = valvelet(
  function request(i) {
    return Promise.resolve(`${i} - ${new Date().toISOString()}`);
  },
  2,
  1000
);

function log(data) {
  console.log(data);
}

for (let i = 0; i < 10; i++) {
  get(i).then(log);
}

/*
0 - 2016-06-02T20:07:33.843Z
1 - 2016-06-02T20:07:33.844Z
2 - 2016-06-02T20:07:34.846Z
3 - 2016-06-02T20:07:34.846Z
4 - 2016-06-02T20:07:35.846Z
5 - 2016-06-02T20:07:35.846Z
6 - 2016-06-02T20:07:36.848Z
7 - 2016-06-02T20:07:36.848Z
8 - 2016-06-02T20:07:37.851Z
9 - 2016-06-02T20:07:37.851Z
*/

This module is not a complete solution if you are trying to throttle your requests to a remote API, but have multiple Node.js processes on the same or multiple hosts, since the state is not shared between the services. That case can be addressed by allowing each process to send up to only a fraction of the total limit. Ex: If you have 4 processes, let each process send up to $limit/4.

MIT