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<array.length; i++) {  var previous=array[i-1]; var current=array[i]; var next=array[i+1];  } 

I need to get the previous and next elements in an unlimited cycle. For example,

The previous element of the first element in the array is the array last element The next element of the last element in the array is the array first element 

What can be the most efficient way to do this. The only way I can think of is to check if the element is the first or last in the array in every round.

In fact, I hope to make the array a closed cycle somehow, rather than linear.

Answer 1

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 2

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; } 

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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; 

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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;