Question

In the following code sample i get a strange behavior

var data = ['xxx', 'yyy'];
for (var i in data)
{
    var a = i;
    var b = data[i];
}

The two first iterations works just fine. I get index "0" and "1" in i, but then it loops one extra time and now the i is "sum". Is this by design or what is this extra iteration used for? The result in my case is always empty and it messes up my code. Is there a way to not do his extra loop?

BR
Andreas

Answer 1

You are looping through an Array, not through an Object. For arrays it’s better to use:

for (var i=0; i<data.length; i=i+1){
  /* ... */
}

In your loop every property of the Array object is taken into account. That makes the for ... in loop for array less predictable. In your case it looks like sum is a property (method) that’s added to Array.prototype elsewhere in your code.

There are more ways to loop through arrays. See for example this SO-question, or this one

Just for fun, a more esoteric way to loop an array:

Array.prototype.loop = function(fn){
  var t = this;
  return (function loop(fn,i){
   return i ? loop(fn,i-1).concat(fn(t[i-1])) : [];
  }(fn,t.length));
}
//e.g.
//add 1 to every value
var a = [1,2,3,4,5].loop(function(val){return val+1;});
alert(a); //=> [2,3,4,5,6]
//show every value in console
var b = [1,2,3,4,5].loop(function(val){return console.log(val), val;});