Product Comonad example

prod_comonad.js

const Prod = t => ({
    extract : () => t[1],
                                                                     
    extend : f => Prod([t[0], f([t[0], t[1]])]),                     

    map : f => Prod([t[0], f(t[1])]),                                
                                                                     
    duplicate : () => Prod([t[0], Prod(t)])
}); 
    
Prod.of = function(t) {
    return Prod(t);                                                  
};  

const Student = {                                                    
    id: 5,
    name: "markus",                                                  
    surname: "pfundstein",                                           
    domain: "hotbox.com"
}

const result = 
    Prod.of([Student, Student])
        .map(({name, surname, domain}) => `${name}.${surname}@${domain}`)     
        .extend(([student, mail]) => `${student.id}, ${mail}`)
        .extract();

console.log(result);

This is a good example to get a first intuition what a comonad is. However, there is a little mistake in your monadic of definition. of is intended to put a value in a minimal context. For a 2-tuple the only sane definition is of = x => [x, x], which simplifies Prod.of([Student, Student]) into Prod.of(Student).