Detect Pangram.js

// A pangram is a sentence that contains every single letter of the alphabet at least once. For example, the sentence "The quick brown fox jumps over the lazy dog" is a pangram, because it uses the letters A-Z at least once (case is irrelevant).

// Given a string, detect whether or not it is a pangram. Return True if it is, False if not. Ignore numbers and punctuation.

function isPangram(string){
  let letters = new Set()
  
  for (let i = 0; i < string.length; i++){
    let char = string[i].toLowerCase()
    if (char >= 'a' && char <= 'z'){
    letters.add(char)
   }
  }
  return letters.size === 26
}