The difference between two array iterations

Question

Im just wondering the difference between these array iterations, and why the second one seems to be really rarely used, is something wrong with it?

var items = [ /*...*/ ]
for (var i = 0; i < items.length; i++) {
    var item = items[i];
    // Do some stuff with the item
}

The second way:

var items = [ /*...*/ ]
for (var i, item; item = items[i]; i++) {
    // Do some stuff with the item
}

Answer 1

The first one is guaranteed to always iterate through every element.

The second one would stop midway if it encounters some false-like element, such as 0.

Answer 2

In the second for loop you need to initialize your i variable.

Consider:

var items = ["string", false, null, 0, "", {}, []];

First loop will loop through the entire array. However, second loop's conditional will evaluate the value assigned to item. The boolean value of this portion would be:

!!(item = items[i])

valid values such as null, 0, false, ""(empty string) and undefined will evaluate to false and break. In the array above, you'll break out of the for loop when item is assigned to false

Answer 3

In second case the termination condition is somewhat javascript-specific - loop terminates when assignment converted to boolean yields false, which happens also on reading array element out of its bounds. This is contrary to other popular languages, where out-of-bouns access to array causes some kind of error and does not guarantee the correct condition for terminating for loop. When any programmer with background in other languages than javascript writes iterating loop, he/she probably avoids patters which he knows are incorrect or even dangerous in these languages.

This would be main reason, I think. But other thing is, like mentioned above, that this condition might be false even before reaching end of iterated array - for example when assigned element has value of false, 0, or any other value, which in javascript implicitly converts to false.