A palindrome is a word, phrase, number, or sequence of words that reads the same backward as forward. An example of a palindrome is "madam", which reads as "madam" backwards. Sentences can be palindromes as well. For example: "taco cat" and "red rum, sir, is murder".

Example were we want to include spaces and punctuation in determining if the string is a palindrome.

palindrome('abba') // true
palindrome('abcdefg !') // false

Methods

Direct Comparison with Reverse

function palindrome(str) {
  let newStr = str.split('').reverse().join('')
  return str === newStr
}

Every() method

Can use the every method to check if every element in the array is true.

every((element, index, array) => {
  /* ... */
})

The every() method tests whether all elements in the array pass the test implemented by the provided function. It returns a Boolean value.

const array = [1, 30, 39, 29, 10, 13]

array.every((value) => value < 40) // true, because all values in the array are less than 40.

We could use every() to check if every element in the array is true. However, the downside if you are doing a lot of extra comparisons. You only NEED to compare until you get halfway through the array.

function palindrome(str) {
  return str.split('').every((character, index) => {
    return character === str[str.length - index - 1]
  })
}

With Spread Operator

const palindrome = (str) => {
  const s = str.toLowerCase().replace(/[\W_]/g, '')
  return s === [...s].reverse().join('')
}

Reverse an integer

const reverseInt = (num) => {
  let reversed = Math.abs(num).toString().split('').reverse().join('')
  console.log(reversed)

  return Math.sign(num) * parseInt(reversed)
}

maxChar

Return the most common character in a string.

const maxChar = (str) => {
  // create an object to store frequency of each character
  let result = {}

  // iterate through the string
  // use regex to remove spaces, lowercase all characters
  for (let char of str.toLowerCase().replace(/[\s]/g, '')) {
    // if it does not exists, set its value to 1
    if (!result[char]) {
      result[char] = 1
    }

    // if the character exists in the object, +1 to its value
    result[char]++
  }

  // now have an object with keys of each char in the string
  // and values of their respective frequency

  // return the key with the highest value
  let highestVal = 0
  let highestValsKey

  for (let key in result) {
    if (result[key] > highestVal) {
      highestVal = result[key]
      highestValsKey = key
    }
  }

  return highestValsKey
}

maxChar('abccc   ccccd') // === // "c"
maxChar('apple 1231111') // === "1"