mapLimit via callbacks

mapLimit.js

function mapLimit (col, limit, mapFunc, doneFunc, newCol = [], index = { value: 0 }, freeThreads = { value: limit }) {
  if (freeThreads.value > 0 && index.value < col.length) {
    // Save current index
    const curIndex = index.value
    // Now the number of free threads is lower on 1
    freeThreads.value -= 1
    mapFunc((err, newItem) => {
      if (err) {
        // Smth bad happend, we call doneFunc with err
        doneFunc(err, newCol)
      } else {
        newCol[curIndex] = newItem
        // We can now free one thread
        freeThreads.value += 1
        if (curIndex === col.length - 1) {
          // If we processed all elements, we can call doneFunc
          doneFunc(null, newCol)
        } else if (freeThreads.value > 0) {
          // if we have free threads, we can call again mapLimit with increased value of index on 1
          index.value += 1
          mapLimit(col, limit, mapFunc, doneFunc, newCol, index, freeThreads)
        }
      }
    }, col[curIndex])
    if (freeThreads.value > 0  && curIndex < col.length) { 
      // if we have free threads, we can call again mapLimit with increased value of index on 1
      index.value += 1
      mapLimit(col, limit, mapFunc, doneFunc, newCol, index, freeThreads) 
    }
  }
}

// Let's test this solution now

const startDate = new Date()

// Let's create a collection
let col = []
for (let i = 0; i < 16; i++) {
  col.push(i)
}
console.log('Input collection is ', col) // [ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 ] 

// Our mapFunc
function makeNumbersNegativeAsyncCall(cb, item) {
  setTimeout(function() {
    newItem = item * -1
    cb(null, newItem)
  }, 200);
}

// Let's try with limit = 2
mapLimit(col, 2, makeNumbersNegativeAsyncCall, (err, newCol) => {
  console.log(`Solution with limit = 2, time is ${new Date().getTime() - startDate.getTime()}ms and output collection is`, newCol) // 1623ms
})

// Let's try now with limit = 8, should be faster
mapLimit(col, 8, makeNumbersNegativeAsyncCall, (err, newCol) => {
  console.log(`Solution with limit = 8, time is ${new Date().getTime() - startDate.getTime()}ms and output collection is`, newCol) // 411ms; yes, it's faster
})

output.md

Input collection is  [ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 ]
Solution with limit = 8, time is 411ms and output collection is [ -0, -1, -2, -3, -4, -5, -6, -7, -8, -9, -10, -11, -12, -13, -14, -15 ]
Solution with limit = 2, time is 1623ms and output collection is [ -0, -1, -2, -3, -4, -5, -6, -7, -8, -9, -10, -11, -12, -13, -14, -15 ]