How to get the previous and next elements of an array loop in JavaScript?

Question

In a simple array loop of Javacript as

for (var i=0; i

        

Answer 1

you need reduce; a built sweet function to get previous and next values of array

[0, 1, 2, 3, 4].reduce(function(previousValue, currentValue, currentIndex, array) {
  return previousValue + currentValue;
});

Answer 2

To add to @Denys answer - this is how you can create a reuseable function

var theArray = [0, 1, 2, 3, 4, 5];
var currentIndex = 0;

function getAtIndex(i) {
    if (i === 0) {
        return theArray[currentIndex];
    } else if (i < 0) {
        return theArray[(currentIndex + theArray.length + i) % theArray.length];
    } else if (i > 0) {
        return theArray[(currentIndex + i) % theArray.length];
    }
}

// usage
getAtIndex(-2)

// you can even go crazy and it still works
getAtIndex(500)

Demo jsfiddle

Answer 3

Use modulus :

var len = array.length;

var current = array[i];
var previous = array[(i+len-1)%len];
var next = array[(i+1)%len];

Note the +len when getting the previous: the reason we need this is to avoid negative indexes, due to the way modulus works (very unfortunately, -x% is -(x%))

Answer 4

as you're talking about "unlimited cycle" I assume your loop is something like that

var i = 0,
    l = array.length;

while( true ) // keep looping
{
    if(i >= l) i = 0;

    // the loop block

    if(/* something to cause the loop to end */) break; // <-- this let execution exit the loop immediately

    i+=1;
}

---

The most efficient way to achieve your goal is the naive one: checking

    var previous=array[i==0?array.length-1:i-1];
    var current=array[i];
    var next=array[i==array.length-1?0:i+1];

obviously cache the length of the array in a variable

var l = array.length;

and (better style) the "vars" out of the cycle

var previuos,
    current,
    next;

---

Note that if you are accessing the array read only there would be a faster (but somewhat strange) way:

l = array.length;
array[-1] = array[l-1]; // this is legal
array[l] = array[0];

for(i = 0; i < l; i++)
{
    previous = array[i-1];
    current = array[i];
    next = array[i+1];
}

// restore the array

array.pop(); 
array[-1] = null;