Declarative style implementation for CodeWars exercise https://www.codewars.com/kata/take-a-number-and-sum-its-digits-raised-to-the-consecutive-powers-and-dot-dot-dot-eureka/

sumDigPow-declarative.js

function sumDigPow(start, stop) {
  // create array from start to stop
  const arr = Array.apply(null, Array(stop-start)).map((_,i) => i+start);
  
  function calc(num) {
    const n_arr = String(num).split('').map(Number).map((n,i) => Math.pow(n,i+1));
    return n_arr.reduce((acc,n) => acc+n);
  }
  
  // create new array with numbers that match criteria
  return arr.filter((n) => calc(n) === n ? true : false);
}

console.log(sumDigPow(0, 150)); // [ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 89, 135 ]