How to check if character is a letter in Javascript?

Question

I am extracting a character in a Javascript string with:

var first = str.charAt(0);

and I would like to check whether it is a letter. Stran

Answer 1

// to check if the given string contain alphabets    
function isPangram(sentence){
        let lowerCased = sentence.toLowerCase();
        let letters = "abcdefghijklmnopqrstuvwxyz";
        // traditional for loop can also be used
        for (let char of letters){
            if (!lowerCased.includes(char)) return false;
        }
        return true;
    }