CODEWITHSUNDEEP
javascriptfor-loop
philomath
philomath

Reputation: 2211

Simple JS For-loop confusion

In this example, I want to remove all the pararaphes. But as you can see, the for-loop below didn't remove them all, it still left the first paragraph for me. http://codepen.io/vinhnghi223/pen/jnkzh

article=document.getElementsByTagName("article")[0];

for (i=0; i<article.childNodes.length; i++) {
 article.removeChild(article.lastChild);
}

However, if I change the code to i<4 (which is less than article.childNodes.length), which returns 5, it works.

I got why article.childNodes.length returns 5. It just confuses me because I think if it works with 4 then it should work with 5.

Upvotes: 0

Views: 102

Answers (2)

Guffa
Guffa

Reputation: 700192

That's because you are changing the collection that you are looping through. You are looping from zero and up, and at the same time you are removing child nodes so that the upper limit of the loop is getting lower. In the middle your loop counter will meat the shrinking number of child nodes, and the loop ends.

For that loop to work, you would need to preserve the initial number of child nodes:

var cnt = article.childNodes.length;
for (i = 0; i < cnt; i++) {
  article.removeChild(article.lastChild);
}

However, you can just loop while there are any child nodes left:

while (article.childNodes.length> 0) {
  article.removeChild(article.lastChild);
}

Upvotes: 2

Blue Skies
Blue Skies

Reputation: 2975

It's because the .childNodes collection updates with every removal. So the .length is shrinking on every iteration, while i is growing. Since you're removing the .lastChild, you're going to end up meeting in the middle, and so the first half won't be removed.

When you hard code the original length, you eliminate the problem.

If your intent is to remove all the children, then just have the condition check to see if a .lastChild exists.

while (article.lastChild) {
    article.removeChild(article.lastChild);
}

"Live" node lists like this can be tricky, so you always need to be careful when modifying their elements in a loop.

Upvotes: 4

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