JSFiddle - React, Tailwind, and code Playground

JavaScript

var ALPHABET = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ';
var input = 'ABOZ';

function testVal(str, constant){ 
  var matchFlag = true;
  var strSplit = str.split("");
  for(var i=0; i<strSplit.length;i++){
    if(constant.indexOf(strSplit[i]) == -1){
       matchFlag = false;
    }
  }
  return matchFlag;
}
alert(testVal(input, ALPHABET)); //TRUE