Square a set of numbers at a time

promise-square.js

const sliceTodos = todos =>
  todos.length <= 3
    ? [todos, []]
    : [todos.slice(0, 3), todos.slice(3)]

const square = (x, callback) =>
  setTimeout(() => callback(x ** 2), 1000)

const squarePromise = x =>
  new Promise(resolve => square(x, res => resolve(res)))

const squareParallel = todos => {
  const [currentTodos, nextTodos] = sliceTodos(todos)

  return Promise.all(
    currentTodos.map(todo => squarePromise(todo))
  )
  .then(results => {
    console.log(`Got results for ${currentTodos} => ${results}`)
    nextTodos.length
      ? squareParallel(nextTodos)
      : console.log('All items squared!')
  })
}

console.log("==== With 8 elements ====")

squareParallel([1, 2, 3, 4, 5, 6, 7, 8])