What is the best way to limit concurrency when using ES6's Promise.all()?

Question

I have some code that is iterating over a list that was queried out of a database and making an HTTP request for each element in that list. That list can sometimes be a rea

Answer 1

expanding on the answer posted by @deceleratedcaviar, I created a 'batch' utility function that takes as argument: array of values, concurrency limit and processing function. Yes I realize that using Promise.all this way is more akin to batch processing vs true concurrency, but if the goal is to limit excessive number of HTTP calls at one time I go with this approach due to its simplicity and no need for external library.

async function batch(o) {
  let arr = o.arr
  let resp = []
  while (arr.length) {
    let subset = arr.splice(0, o.limit)
    let results = await Promise.all(subset.map(o.process))
    resp.push(results)
  }
  return [].concat.apply([], resp)
}

let arr = []
for (let i = 0; i < 250; i++) { arr.push(i) }

async function calc(val) { return val * 100 }

(async () => {
  let resp = await batch({
    arr: arr,
    limit: 100,
    process: calc
  })
  console.log(resp)
})();