conc.js

// example 1

const p1 = new Promise(resolve => setTimeout(resolve, 300, 'first promise'))
const p2 = new Promise(resolve => setTimeout(resolve, 200, 'second promise'))
const p3 = new Promise(resolve => setTimeout(resolve, 700, 'third promise'))
const p4 = new Promise(resolve => setTimeout(resolve, 500, 'fourth promise'))
const p5 = new Promise(resolve => setTimeout(resolve, 900, 'fifth promise'))

function concurrentCall(arrQueries, limit) {
  const sortRes = []
  let count = 0
  let totalCount = arrQueries.length

  return new Promise(resolve => {
    function handlePromise(prom, index) {
      return fetch(prom)
        .then(res => res.json())
        .then(data => {
          sortRes[index] = data
          count++
          if (count >= totalCount) {
            resolve(sortRes)
          }
        })
    }
    arrQueries.reduce((acc, prom, index) => {
      if (index < limit) {
        console.log(index)
        return acc.then(handlePromise(prom, index))
      }

      return acc.then(() => handlePromise(prom, index))
    }, Promise.resolve())
  })
}
const g = [
  'https://jsonplaceholder.typicode.com/todos/1',
  'https://jsonplaceholder.typicode.com/todos/2',
  'https://jsonplaceholder.typicode.com/todos/3',
  'https://jsonplaceholder.typicode.com/todos/4',
  'https://jsonplaceholder.typicode.com/todos/5'
]
concurrentCall(g, 4).then(res => console.log(res))


// example 2

var source = [
  { url: 'http://abc/1', time: 1000, value: 1 },
  { url: 'http://abc/2', time: 500, value: 2 },
  { url: 'http://abc/3', time: 300, value: 3 },
  { url: 'http://abc/4', time: 300, value: 4 },
  { url: 'http://abc/5', time: 100, value: 5 }
]

function handle(sources, limit) {
  var limit = 0
  var index = limit
  var response = []
  var totallimit = sources.length

  return new Promise(resolve => {
    for (var i = 0; i < limit; i++) {
      getNext(i)
    }

    function getNext(i) {
      var item = sources[i]

      if (!item) {
        console.log('everything in work right now')
        return
      }

      setTimeout(() => {
        console.log(item.value)
        response[i] = item.value
        limit++

        if (limit >= totallimit) {
          console.log('have response')

          resolve(response)
        } else {
          var nextIndex = index
          index++
          getNext(nextIndex)
        }
      }, item.time)
    }
  })
}

handle(source, 5).then(data => {
  console.log(data)
})