Check for Palindromes [looking for bugs]

Hello! I was solving this task and suddenly my code sometimes works well and sometimes not. I can’t find the reason. Help me please. Here is my code.
P.S. I haven’t looked at the solution.
P.P.S. I know I should have named variables more clear and created a new function for the nested foor loop.

Your code so far

function palindrome(str) {
  var letter = ['a', 'b', 'c', 'd', 'e', 'f', 'g',
          'h', 'i', 'j', 'k', 'l', 'm', 'n',
          'o', 'p', 'q', 'r', 's', 't', 'u',
          'v', 'w', 'x', 'y', 'z'
             ];//characters a palindrome can contain
  var arrStr = [];//array for str
  var strReversed = [];//array for reversed str
  arrStr = str.split("");
  for(var i = 0; i < arrStr; i++)
    for(var j = 0; j < word.length; j++)
      if(arrStr[i] == letter[j])//check if it isn't a letter
        delete arrStr[i];//if so, delete the element
  strReversed = arrStr;//now we can see if there's
  strReversed.reverse();//a difference between them
  for(var k = 0; k < arrStr.length; k++)
    if(arrStr[k] != strReversed[k])//once we find some characters do not match
      return false;//it means str isn't a palindrome
  return true;


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Link to the challenge:

You shouldn’t be deleting digits. You are probably failing the “1 eye for 1 eye” test.

Also: The toLowerCase method returns the value of the string converted to lowercase. toLowerCase does not affect the value of the string itself.