JSFiddle - React, Tailwind, and code Playground

JavaScript

Array.prototype.ucfirst = function() 
{
    var a = this;
    for( i = 0; i < a.length; i++ )
    {
        // проверки типа a[i] нет:
        a[i] = a[i].ucfirst();
    }
    return a;
}
alert( ['LALAlala','ZaZaZa'].ucfirst() );