Palindromes in JavaScript

check-chars-iterating-from-beginning-to-middle.js

// https://www.freecodecamp.com/challenges/check-for-palindromes#?solution=function%20palindrome(str)%20%7B%0A%20%20str%20%3D%20str.toLowerCase().replace(%2F%5B%5Ea-z0-9%5D%2Fg%2C%20%22%22)%3B%0A%20%20for(var%20i%20%3D%200%3B%20i%20%3C%20str.length%2F2%3B%20%2B%2Bi)%20%0A%20%20%20%20if(str%5Bi%5D%20!%3D%3D%20str%5Bstr.length-i-1%5D)%0A%20%20%20%20%20%20return%20false%3B%0A%20%20return%20true%3B%0A%7D%0A%0A%0A%0Apalindrome(%22eye%22)%3B%0A
function palindrome(str) {
  str = str.toLowerCase().replace(/[^a-z0-9]/g, "");
  for(var i = 0; i < str.length/2; ++i) {
    if(str[i] !== str[str.length-i-1]) {
      return false;
    }
  }
  return true;
}

console.log(palindrome("My age is 0, 0 si ega ym."));
// start
"My age is 0, 0 si ega ym."

// after toLowerCase
"my age is 0, 0 si ega ym."

// after after replace
"myageis00siegaym"

// i goes from 0 to 7
// m              m
  "myageis00siegaym"
//  y            y
  "myageis00siegaym"
//   a          a
  "myageis00siegaym"
//    g        g
  "myageis00siegaym"
// ...
//        00
  "myageis00siegaym"

counting-from-beginning-and-end.js

function shouldCollapse(char) {
  // if it's outside of a-z, A-Z, 0-9 then collapse it
  return (char < "a" || "z" < char) &&
         (char < "A" || "Z" < char) &&
         (char < "0" || "9" < char);
}

function mismatch(char1, char2) {
  return char1.toLowerCase() !== char2.toLowerCase();
}

function palindrome(str) {
  var start = -1;
  var end   = str.length;
  while(start < end) {
    do { start += 1; } while (shouldCollapse(str[start]));
    do { end   -= 1; } while (shouldCollapse(str[end]));
    if(mismatch(str[start], str[end])) {
      return false;
    }
  }
  return true;
}

function testPalindrome(expected, string) {
  var pass = (expected === palindrome(string));
  console.log(pass ? "pass" : "fail");
}

testPalindrome(true,  "eye");
testPalindrome(true,  "race car");
testPalindrome(false, "not a palindrome");
testPalindrome(true,  "A man, a plan, a canal. Panama");
testPalindrome(true,  "never odd or even");
testPalindrome(false, "nope");
testPalindrome(false, "almostomla");
testPalindrome(true,  "My age is 0, 0 si ega ym.");
testPalindrome(false, "1 eye for of 1 eye.");
testPalindrome(true,  "0_0 (: /-\ :) 0-0");

find-matches-instead-of-removing-nonmatches.js

function palindrome(str) {
  // instead of removing things we don't want, we'll find things we do want
  collapsedStr = str.toLowerCase().match(/[a-z0-9]/g).join();

  // is it the same as its reverse?
  reversedStr  = collapsedStr.split("").reverse().join("");
  return collapsedStr === reversedStr;
}

function testPalindrome(expected, string) {
  var pass = (expected === palindrome(string));
  console.log(pass ? "pass" : "fail");
}

testPalindrome(true,  "eye");
testPalindrome(true,  "race car");
testPalindrome(false, "not a palindrome");
testPalindrome(true,  "A man, a plan, a canal. Panama");
testPalindrome(true,  "never odd or even");
testPalindrome(false, "nope");
testPalindrome(false, "almostomla");
testPalindrome(true,  "My age is 0, 0 si ega ym.");
testPalindrome(false, "1 eye for of 1 eye.");
testPalindrome(true,  "0_0 (: /-\ :) 0-0");

str-matches-its-reverse.js

function palindrome(str) {
  // instead of removing things we don't want, we'll find things we do want
  collapsedStr = str.toLowerCase().match(/[a-z0-9]/g).join();

  // is it the same as its reverse?
  reversedStr  = collapsedStr.split("").reverse().join("");
  return collapsedStr === reversedStr;
}

function testPalindrome(expected, string) {
  var pass = (expected === palindrome(string));
  console.log(pass ? "pass" : "fail");
}

testPalindrome(true,  "eye");
testPalindrome(true,  "race car");
testPalindrome(false, "not a palindrome");
testPalindrome(true,  "A man, a plan, a canal. Panama");
testPalindrome(true,  "never odd or even");
testPalindrome(false, "nope");
testPalindrome(false, "almostomla");
testPalindrome(true,  "My age is 0, 0 si ega ym.");
testPalindrome(false, "1 eye for of 1 eye.");
testPalindrome(true,  "0_0 (: /-\ :) 0-0");

using-filter.js

function palindrome(str) {
  var isAlphaNum = function(char) {
    return ("a" <= char && char <= "z") || ("0" <= char && char <= "9");
  }
  var filtered = str.toLowerCase().split("").filter(isAlphaNum).join("");
  var reversed = filtered.split("").reverse().join("");
  return filtered === reversed;
}

function testPalindrome(expected, string) {
  var pass = (expected === palindrome(string));
  console.log(pass ? "pass" : "fail");
}

testPalindrome(true,  "eye");
testPalindrome(true,  "race car");
testPalindrome(false, "not a palindrome");
testPalindrome(true,  "A man, a plan, a canal. Panama");
testPalindrome(true,  "never odd or even");
testPalindrome(false, "nope");
testPalindrome(false, "almostomla");
testPalindrome(true,  "My age is 0, 0 si ega ym.");
testPalindrome(false, "1 eye for of 1 eye.");
testPalindrome(true,  "0_0 (: /-\ :) 0-0");