Insert an element between array elements

insertBtwn.js

/*
 * Insert insertedEl or the result of insertedEl (if it is a function) between the elements of arr.
 * Attention: nothing is inserted before the first element or after the last element of arr.
 * If the predicate function is defined, the function will instert the value only if the predicate is verified (returns a truthy value).
 *
 * insertedEl and predicate function are both called with these arguments: prev, curr, tIdx and tArr.
 * predicate has one more argument as last which is called value.
 * where prev = the accumulator array wich contains the current partial value of the final array or []
 *       curr = the current element that will be pushed in the prev array
 *       tIdx = index of curr in the original array
 *       tArr = the original array
 *       value (only for predicate) = the value returned by insertedEl
 */
function insertbtwn(arr, insertedEl, predicate) {
  const insertion = (typeof insertedEl === 'function') ? insertedEl : function (prec, curr, tIdx, tArr) { return insertedEl; };
  return arr.reduce( (prec, curr, tIdx, tArr) => {
    if (tIdx > 0) {
      let value = insertion(prec, curr, tIdx, tArr);
      if (predicate && typeof(predicate) === 'function') {
        if (predicate(prec, curr, tIdx, tArr, value)) {
          prec.push(value);
        }
      } else { prec.push(value); }
    }
    prec.push(curr);
    return prec;
  }, []);
}