javascript - Functional way to create an array of numbers

Question

How could I write the following code more functionally using ES6, without any 3rd party libraries?

// sample pager array
// * output up to 11 pages
// * the current page in the middle, if page > 5
// * don't include pager < 1 or pager > lastPage
// * Expected output using example:
//     [9,10,11,12,13,14,15,16,17,18,19]

const page = 14 // by example
const lastPage = 40 // by example
const pagerPages = page => {
  let newArray = []
  for (let i = page - 5; i <= page + 5; i++) {
    i >= 1 && i <= lastPage ? newArray.push(i) : null
  }
  return newArray
}

I would like to avoid Array.push, and possibly the for loop, but I'm not sure how I would achieve it in this situation.

Answer 1

  const pageRange = (lastPage, page) => ((start, end) => Array.from({length: end - start + 1}, (_,i) => i + start))(Math.max(1, page - 5), Math.min(lastPage, page + 5));
 const newArray = pageRange(40, 14);

This is a purely functional approach. It uses Math.max/min to achieve the boundaries and then uses an IIFE to pass these boundaries to Array.from which will create an array of end - start elements and every of these elements will be the position in the array increased by the startvalue.

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PS: IMO your code is actually much more concise (except from that unneccessary ternary) and far more readable than mys, just saying...