jsonpath/lib/slice.js

module.exports = function(arr, start, end, step) {

  if (typeof start == 'string') throw new Error("start cannot be a string");
  if (typeof end == 'string') throw new Error("end cannot be a string");
  if (typeof step == 'string') throw new Error("step cannot be a string");

  var len = arr.length;

  if (step === 0) throw new Error("step cannot be zero");
  step = step ? integer(step) : 1;

  // normalize negative values
  start = start < 0 ? len + start : start;
  end = end < 0 ? len + end : end;

  // default extents to extents
  start = integer(start === 0 ? 0 : !start ? (step > 0 ? 0 : len - 1) : start);
  end = integer(end === 0 ? 0 : !end ? (step > 0 ? len : -1) : end);

  // clamp extents
  start = step > 0 ? Math.max(0, start) : Math.min(len, start);
  end = step > 0 ? Math.min(end, len) : Math.max(-1, end);

  // return empty if extents are backwards
  if (step > 0 && end <= start) return [];
  if (step < 0 && start <= end) return [];

  var result = [];

  for (var i = start; i != end; i += step) {
    if ((step < 0 && i <= end) || (step > 0 && i >= end)) break;
    result.push(arr[i]);
  }

  return result;
}

function integer(val) {
  return String(val).match(/^[0-9]+$/) ? parseInt(val) :
    Number.isFinite(val) ? parseInt(val, 10) : 0;
}