Alphabetic SubSequence
Check whether the given string is a subsequence of the plaintext alphabet.

Example

• For s = “effg” or s = “cdce”, the output should be
  alphabetSubsequence(s) = false
  

• For s = “ace” or s = “bxz”, the output should be
  alphabetSubsequence(s) = true.
  

function alphabetSubsequence(s) {
  const chars = s.split('');
  const charValues = []

  chars.forEach(char => {
    charValues.push(char.charCodeAt(0))
  });
  if (new Set(charValues).size !== charValues.length) {
    return false
  }
  for (let i = 0; i < charValues.length - 1; i++) {
    if (charValues[i] >= charValues[i + 1]) {
      return false
    }
    }
  return true

}

console.log(alphabetSubsequence('zab')) false 
console.log(alphabetSubsequence('effg')) false 
console.log(alphabetSubsequence('cdce')) false
console.log(alphabetSubsequence('ace')) true
console.log(alphabetSubsequence('bxz')) true

We create function called alphabetSubsequence with parameter s and local variable chars = s.split(‘’) and charValues = []

The split() method divides a String into an ordered list of substrings, puts these substrings into an array, and returns the array. The division is done by searching for a pattern; where the pattern is provided as the first parameter in the method’s call. - MDN

We do a forEach loop on chars array and with callback function and parameter char we push charValues.push(char.charCodeAt(0)

The charCodeAt() method returns an integer between 0 and 65535 representing the UTF-16 code unit at the given index. - MDN

If we use Set function to delete any duplicates new Set(charValues).size !== charValues.length if true we return false

We loop again with for loop this time to see the starting value (0) and until the last remainding value charValues.lenght - 1 and we increment by 1

Inside the for loop we check if charValues[i] >= charValues[i + 1] and if it is we return false again.

So if all the cases above are false we return true