getInitials prototype

function Person( first, last ) {
    this.firstName = first;
    this.lastName = last;
}
Person.prototype.getInitials = function() {
    let firstInitial = this.firstName.substring( 0, 1 ).toUpperCase();
    let lastInitial = this.lastName.substring( 0 , 1 ).toUpperCase();
    
    return firstInitial + lastInitial;
};