JSFiddle - React, Tailwind, and code Playground

JavaScript

var fullnames = ["Karri", "Ismo", "Grigori", "Ahmed", "Roope", "Arto"];
var counts = {};

for (var i = 0; i < fullnames.length; i++)
{
    if (!counts.hasOwnProperty(fullnames[i]))
    {
        counts[fullnames[i]] = 1;
    }
    else 
    {
        counts[fullnames[i]]++;
    }
}

console.log(counts);