谁能解释为什么我在Javascript中遇到此错误？ [无法读取未定义的属性'length']

Question

Here is my code:

function isEven(num) {;
  if (num % 2 === 0) {
  return true;}
  else {
  return false;}
  }

function convertToIntegers(lst) {
  lst.split(' ');
  var len = lst.length;
  var count = 0;
  while (count < len) {
      lst[count] = parseInt(lst[count]);
      count++}
      }




function iqTest(numbers){

  var int_list = convertToIntegers(numbers);
  var e_list = [];
  var o_list = [];
  var count = 0;
  var len_int = int_list.length;
  while (count < len_int) {
      if (isEven(int_list[count]) === true) {
      e_list.push(count);}
      else if (isEven(count) === false) {
      o_list.push(count);}
      count++}
  if (e_list.length < 2 && e_list.length > 0 && o_list.length > 2) {
      return e_list[0];}
  else if (o_list.length < 2 && o_list.length > 0 && e_list.length > 2) {
      return o_list[0];}
      }

Everytime I run it I get this error that says "TypeError: Cannot read property 'length' of undefined at iqTest". Can anyone explain to me how I can fix this error. I don't understand why any object would be undefined in my code. Every var declaration I make is complete. I thought undefined only came up if I wrote something like 'var a' without defining it.

Answer 1

this one is very easy!
in these situations you want to trace the value back.

1. you use .length in iqTest() in the line var len_int = int_list.length;

2. the last assignment to int_list is at line var int_list = convertToIntegers(numbers);

3. glimps at convertToIntegers(numbers) shows no return statement.

therefore conclusion:
convert2Integers() returns nothing into int_list
then you use int_list.length; of undefined

it seems you wish to return lst from convert2Integers()

function convertToIntegers(lst) {
  lst = lst.split(' '); // NOTICE CHANGE, + create a different list!
  var len = lst.length;
  var count = 0;
  while (count < len) {
    lst[count] = parseInt(lst[count]);
    count++
  }
  return lst; //THIS LINE was Missing!
}

Answer 2

Tomer W and Patrick Evans are right. change your helper function

function convertToIntegers(lst) {
  lst = lst.split(' ');
  var len = lst.length;
  var count = 0;
  while (count < len) {
    lst[count] = parseInt(lst[count]);
    count++;
  }
  return lst;
}

this way you are storing th result of the split and then returning the array when done.

that being said this function isn't needed. Check out the built in array function map

Answer 3

To expand on @Patrick Evans comment you need to save the result of the split then use that for your while loop. Just as a side note you know the number of iterations of the loop so you can use a for loop.

function convertToIntegers(lst) {
//lst.split(' ');
var lst_array = lst.split(' ');
var len = lst_array .length;
var count = 0;
while (count < len) {
  lst_array[count] = parseInt(lst_array[count]);
  count++}
  }
//
//for loop code
//
//for(i=0;i<lst_array.length;i++){
//  lst_arry[i] = parseInt(lst_array[i]);
//}