Batch iterator using promises

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121.5.03 years ago7 years agoMinified + gzip package size for batch-iterator in KB


JavaScript batch iterator using promises.
Useful when you need to throttle how many concurrent processes are running simultaneously, while being notified when they're all done (or something fails).


npm install batch-iterator --save


var iterator = require('batch-iterator');

iterator(list, batchSize, promise, data)
  .then(function(accumulator) {
    console.log('List of result from promise', accumulator);
  .catch(function(err) {
    console.log("A promise failed", err);

The list will be iterated, and each item in the list will be passed on to the promise callback.
batchSize is how many items in the list should be started simultaneously. Default size is 10. If set to 0, all array elements are processed at once. If negative values are provided, 'Invalid batchSize' error is returned
The accumulator is a list of the data resolved in each promise.
The data will be passed to each function as additional data.
The index of the array item is accessible as the LAST parameter of the passed function. (also afte 'data', if present)


In the example below we use the Pageres module to capture 100 screenshots in batches of 10.
const Pageres = require('pageres');
const iterator = require('batch-iterator');
const urls = ["", ..., ]; // imagine 100 urls

const startTime =;

iterator(urls, 10, function(url) {
       var pageres = new Pageres();
       // Return the pageres promise
       return pageres
           .src(url, ['1024x768'])
           .dest(process.cwd() + '/screenshots')
   .then(function() {
     const endTime =;
     const diffTime = (endTime - startTime) / 1000;
     console.log("Screenshots captured in " + diffTime + " seconds.");
   .catch(function(err) {