What is the best way to do loops in JavaScript

Question

I have stumbled into several methods of looping in JavaScript, what I like the most is:

for(var i = 0; i < a.length; i++){
    var element = a[i];
}


        

Answer 1

Just store the length in a variable first.

  var len = a.length;
  for (var i = 0; i < len; i++) {
    var element = a[i];
  }

Answer 2

If you have many elements in the array and speed is an issue then you want to use a while loop that iterates from highest to lowest.

  var i = a.length;
  while( --i >= 0 ) {
    var element = a[i];
    // do stuff with element
  }  

Answer 3

I've started using iterators where relevant. Performance is reasonable, however more importantly it allows you to encapsulate the looping logic:

function createIterator(x) {
    var i = 0;

     return function(){
       return x[i++];
    };
}

Then to use:

var iterator=createIterator(['a','b','c','d','e','f','g']);

iterator();

returns "a";

iterator();

returns "b";

and so on.

To iterate the whole list and display each item:

var current;

while(current=iterator())
{
    console.log(current);
}

Be aware that the above is only acceptable for iterating a list that contains "non-falsy" values. If this array contained any of:

• 0
• false
• ""
• null
• NaN

the previous loop would stop at that item, not always what you want/expect.

To avoid this use:

var current;

while((current=iterator())!==undefined)
{
   console.log(current);
}