JavaScript version
isAlpha.js
const isAlpha = (str) => /^[A-Z]+$/i.test(str);TypeScript version
isAlpha.ts
const isAlpha = (str: string): boolean => /^[A-Z]+$/i.test(str);Examples
examples.js
isAlpha('helloworld'); // true
isAlpha('HelloWorld'); // true
isAlpha('hello world'); // false
isAlpha('0123456789'); // false