[Example of Limiting Concurrency using ES6 iterator] #JavaScript

index.js

/* 
Reference: https://stackoverflow.com/questions/40639432/what-is-the-best-way-to-limit-concurrency-when-using-es6s-promise-all

[Symbol.iterator]() is equivalent to .values()
const iterator = [1,2,3][Symbol.iterator]() 
*/
const iterator = [1,2,3].values()

// loop over all items with for..of
for (const x of iterator) {
  console.log('x:', x)
  
  // notices how this loop continues the same iterator
  // and consumes the rest of the iterator, making the
  // outer loop not logging any more x's
  for (const y of iterator) {
    console.log('y:', y)
  }
}

/**
We can use the same iterator and share it across workers.
If you had used .entries() instead of .values() you would have goten a 2D array with [index, value] which i will demonstrate below with a concurrency of 2
**/

const sleep = n => new Promise(rs => setTimeout(rs,n))

async function doWork(iterator) {
  for (let [index, item] of iterator) {
    await sleep(1000)
    console.log(index + ': ' + item)
  }
}

const arr = Array.from('abcdefghij')
const workers = new Array(2).fill(arr.entries()).map(doWork)
//    ^--- starts two workers sharing the same iterator

Promise.all(workers).then(() => console.log('done'))