implementation of map

map.js

function map(arr, op, thisArg) {
  if (typeof op !== 'function') throw new Error('operation must be a function')
  if (!Array.isArray(arr)) throw new Error('first argument must be an array')
  if (typeof thisArg !== 'object' && thisArg !== undefined) throw new Error('`this` argument must be an object')
  var newArr = arr.slice()
  op = op.bind((thisArg || null))
  for (var i = 0, l = newArr.length; i < l; i++) {
    newArr[i] = op(newArr)
  }
  return newArr
}