A veces, necesitamos un ciclo sin fin sobre un array de items, como un carrusel de imágenes o un lista de reproducción de audio. Así es como tomar un array y darle “bucle poderosos”

var aList = ['A','B','C','D','E'];

function make_looper( arr ){

arr.loop_idx = 0;

// return current item
arr.current = function(){

if( this.loop_idx < 0 ){// First verification
this.loop_idx = this.length - 1;// update loop_idx
}

if( this.loop_idx >= this.length ){// second verification
this.loop_idx = 0;// update loop_idx
}

return arr[ this.loop_idx ];//return item
};

// increment loop_idx AND return new current
arr.next = function(){
this.loop_idx++;
return this.current();
};
// decrement loop_idx AND return new current
arr.prev = function(){
this.loop_idx--;
return this.current();
};
}

make_looper( aList);

aList.current();// -> A
aList.next();// -> B
aList.next();// -> C
aList.next();// -> D
aList.next();// -> E
aList.next();// -> A
aList.pop() ;// -> E
aList.prev();// -> D
aList.prev();// -> C
aList.prev();// -> B
aList.prev();// -> A
aList.prev();// -> D

Utilizando el operador % (módulo) es más vistoso. El modulo retorna el resto de la division ( 2 % 5 = 1 y 5 % 5 = 0):

var aList = ['A','B','C','D','E'];

function make_looper( arr ){

arr.loop_idx = 0;

// return current item
arr.current = function(){
this.loop_idx = ( this.loop_idx ) % this.length;// no verification !!
return arr[ this.loop_idx ];
};

// increment loop_idx AND return new current
arr.next = function(){
this.loop_idx++;
return this.current();
};

// decrement loop_idx AND return new current
arr.prev = function(){
this.loop_idx += this.length - 1;
return this.current();
};
}

make_looper( aList);

aList.current();// -> A
aList.next();// -> B
aList.next();// -> C
aList.next();// -> D
aList.next();// -> E
aList.next();// -> A
aList.pop() ;// -> E
aList.prev();// -> D
aList.prev();// -> C
aList.prev();// -> B
aList.prev();// -> A
aList.prev();// -> D