nodejs: Execute multiple tasks in parallel

tasks.js

#!/usr/bin/env node

const async = require('async')

const numTasks = 10
const numParallel = 4

function doTask(i) {
  console.log(`${i} start`)
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      console.log(`${i} end`)
      resolve(i)
    }, 1000)
  })
}

async function run() {
  const tasks = [...Array(numTasks).keys()].map(i => {
    return cb => {
      return doTask(i)
        .then(res => cb(null, res))
        .catch(err => cb(err))
    }
  })

  await new Promise((resolve, reject) => {
    async.parallelLimit(tasks, numParallel, (err, result) => {
      if (err != null) {
        reject(err)
        return
      }

      resolve(result)
    })
  })

  console.log('done')
}

run()