为什么我的回文代码在每种情况下都运行正确？

Question

This code returns true for every scenario; it returns true if it's a palindrome and it returns true if it's not a palindrome.

 function isPalindrome(string){
  string = string.toLowerCase();
  var charactersArr = string.split(' ');
  var validCharacters = 'abcdefghijklmnopqrstuvwxyz'.split(' ');

  var lettersArr = [];
  charactersArr.forEach(char => {
    if (validCharacters.indexOf(char)> - 1) lettersArr.push(char);
  });
  return lettersArr.join(' ') === lettersArr.reverse().join(' ');
}
isPalindrome("Taco Cat");

Answer 1

function palindrome(str) {
 var re = /[^A-Za-z0-9]/g;
 str = str.toLowerCase().replace(re, '');
 var len = str.length;
 for (var i = 0; i < len/2; i++) {
   if (str[i] !== str[len - 1 - i]) {
       return false;
   }
 }
 return true;
}

Use this code for checking palindromes. You are complicating your code by passing in var validCharacters = 'abcdefghijklmnopqrstuvwxyz'.split(' ');You could use re like I have to include all possible combinations including numbers !

Answer 2

It always returns true because lettersArr is always empty. Try to debug your code using debugger or just use console.log for this purpose.

I believe at least this line var charactersArr = string.split(' '); is not doing what is expected: it splits a sentence by space characters into words instead of splitting into separate letters.

Answer 3

It's because lettersArr is always empty.... Try this code:

 function isPalindrome(string) { let reversed = string.toLowerCase().split('').reverse().join(''); return reversed === string.toLowerCase() } let result1 = isPalindrome("Foo bar") let result2 = isPalindrome("mom") console.log(result1, result2)

Answer 4

lettersArr is not what you expect.

You are splitting the input string by spaces, which means you get ['taco', 'cat'] and not ['t', 'a', 'c', 'o', 'c', 'a', 't'] .

Then when this line runs:

if (validCharacters.indexOf(char)> - 1) lettersArr.push(char);

char is 'taco' which isn't found in validCharacters , so nothing gets pushed into lettersArr .

And since lettersArr is empty:

lettersArr.join(' ') === lettersArr.reverse().join(' ');

Becomes:

[].join(' ') === [].reverse().join(' ')

Which, of course, it always true.

---

To fix it you simply want to split the string by an empty string, which gives you array of single character strings.

 function isPalindrome(string){ string = string.toLowerCase(); var charactersArr = string.split(''); var validCharacters = 'abcdefghijklmnopqrstuvwxyz'.split(''); var lettersArr = []; charactersArr.forEach(char => { if (validCharacters.indexOf(char)> - 1) lettersArr.push(char); }); return lettersArr.join(' ') === lettersArr.reverse().join(' '); } console.log("Taco Cat", isPalindrome("Taco Cat")); console.log("abc", isPalindrome("abc"));