Insert an item at a certain index in a recurring number of items inside of an array

insertEvery.js

/**
 * Insert an item (d) at a certain index (b) in a 
 * recurring number (a) of items inside of an array (c)
 * 
 * "For every 20 items in an array, 
 *  insert this at the 12th index in that section"
 * 
 *  var myArray = Array.from({length: 80}, (v, k) => k);
 *  insertItemEvery(20, 12, myArray, "Hello index");
 *
 * @param {number} a - Amount of items per section
 * @param {number} b - Index to insert item at
 * @param {array} c - Array to make inserts in
 * @param {any} d - Item to insert
 */
 
function insertItemEvery(a, b, c, d) {
  var L = c.length;
  var t = Math.floor(L / a) + (L % a >= b ? 1 : 0);
  var T = Math.floor((L + t) / a) + ((L + t) % a >= b ? 1 : 0);
  var C = [];
  
  for (var i = 0, j = 0; i < T; i++) {
    var e = i * a;
    var f = e + b - (i + 1);
    var g = e + a;
    var D = Array.isArray(d) ? d[j++ % d.length] : d;
    
    C = [...C, ...c.slice(e, f), D, ...c.slice(f, g)];
  }
  
  var CL = C.length - T;
  
  return CL < L ? C.concat(c.slice(CL, L)) : C;
}