JSFiddle - React, Tailwind, and code Playground

HTML

<button type="button" onclick="javascript: SingleCharacterString('aaaa');">
  aaaa
</button>

JavaScript

function SingleCharacterString (str)
{
    var Fletter = str.substr(0, 1);
    return (str.replace(new RegExp(Fletter, 'g'), "").length == 0); //Remove all letters that are the first letters, if they are all the same, no letters will remain
}

alert('test ' + SingleCharacterString('test'));
alert('aaaa ' + SingleCharacterString('aaaa'));