怪异的循环Javascript

Question

I have a for loop to loop through the node which is <ul> that has 5 <li> elements. I am trying to create a div for each of those li elements

When I try to append the li elements to div then the for loop is behaving strange

for (var i = 0; i < docNodes.children.length; i++) {
        var docs = document.createElement('div');
        docs.appendChild(docNodes.children[i]); //If this line is removed then it loops 5 times else it loops only 3 times
    }

Could someone let me know what is wrong with the code

Answer 1

The behavior isn't that weird. You're changing the length of docNodes.children in every iteration of the loop. For the next iteration, it checks the length again:

i | Length | i < length
0     5        true
1     4        true
2     3        true
3     2        false // Loop stops before the 4th iteration.

The reason the length is changing, is because docs.appendChild actually removes the target element from it's previous parent.

A way to work around this issue, it to iterate over the children in the opposite order:

for (var i = docNodes.children.length - 1; i >= 0 ; i--) {
    var docs = document.createElement('div');
    docs.appendChild(docNodes.children[i]);
}

i | Length | i >= 0
4     5       true
3     4       true
2     3       true
1     2       true
0     1       true
-1    0       false // Loop stops before the 6th iteration. 

Answer 2

As other point out you modify the length of the array during the loop. In JavaScript it's better to use Array.prototype.forEach instead of an old fashioned for loop.

For example:

docNodes.children.forEach(function (child) {
    var docs = document.createElement('div');
    docs.appendChild(child);
});

Answer 3

Try this. And apply similar logic into your code.

<div id="list"></div>

<script>
let children = ['1','2','3','4','5'];

for (var i = 0; i < children.length; i++) {
  var node = document.createElement("LI");
  var textnode = document.createTextNode(children[i]);
  node.appendChild(textnode);
  document.getElementById("list").appendChild(node);
}
</script>