JSFiddle - React, Tailwind, and code Playground

HTML

<textarea id="output"></textarea>

JavaScript

String.prototype.countLetters = function() 
{
    var arr = new Array(),
        letters = this.split('');
    
    for(var i = 0; i < letters.length; i++)
    {
        var letter = letters[i].toUpperCase();

        if(arr[letter] !== undefined)
        {
            arr[letter]+= 1;
        }
        else
        {
            arr[letter] = 1;
        }
    }
    
    return arr;
}

console.log('Hello'.countLetters());