Why should y.innerHTML = x.innerHTML; be avoided?

Let's say that we have a DIV x on the page and we want to duplicate ("copy-paste") the contents of that DIV into another DIV y. We could do this like so:

y.innerHTML = x.innerHTML;

or with jQuery:

$(y).html( $(x).html() );

However, it appears that this method is not a good idea, and that it should be avoided.

(1) Why should this method be avoided?

(2) How should this be done instead?

Update:
For the sake of this question let's assume that there are no elements with ID's inside the DIV x.
(Sorry I forgot to cover this case in my original question.)

Conclusion:
I have posted my own answer to this question below (as I originally intended). Now, I also planed to accept my own answer :P, but lonesomeday's answer is so amazing that I have to accept it instead.

This method of "copying" HTML elements from one place to another is the result of a misapprehension of what a browser does. Browsers don't keep an HTML document in memory somewhere and repeatedly modify the HTML based on commands from JavaScript.

When a browser first loads a page, it parses the HTML document and turns it into a DOM structure. This is a relationship of objects following a W3C standard (well, mostly...). The original HTML is from then on completely redundant. The browser doesn't care what the original HTML structure was; its understanding of the web page is the DOM structure that was created from it. If your HTML markup was incorrect/invalid, it will be corrected in some way by the web browser; the DOM structure will not contain the invalid code in any way.

Basically, HTML should be treated as a way of serialising a DOM structure to be passed over the internet or stored in a file locally.

It should not, therefore, be used for modifying an existing web page. The DOM (Document Object Model) has a system for changing the content of a page. This is based on the relationship of nodes, not on the HTML serialisation. So when you add an li to a ul, you have these two options (assuming ul is the list element):

// option 1: innerHTML
ul.innerHTML += '<li>foobar</li>';

// option 2: DOM manipulation
var li = document.createElement('li');
li.appendChild(document.createTextNode('foobar'));
ul.appendChild(li);

Now, the first option looks a lot simpler, but this is only because the browser has abstracted a lot away for you: internally, the browser has to convert the element's children to a string, then append some content, then convert the string back to a DOM structure. The second option corresponds to the browser's native understanding of what's going on.

The second major consideration is to think about the limitations of HTML. When you think about a webpage, not everything relevant to the element can be serialised to HTML. For instance, event handlers bound with x.onclick = function(); or x.addEventListener(...) won't be replicated in innerHTML, so they won't be copied across. So the new elements in y won't have the event listeners. This probably isn't what you want.

So the way around this is to work with the native DOM methods:

for (var i = 0; i < x.childNodes.length; i++) {
    y.appendChild(x.childNodes[i].cloneNode(true));
}

Reading the MDN documentation will probably help to understand this way of doing things:

• appendChild
• cloneNode
• childNodes

Now the problem with this (as with option 2 in the code example above) is that it is very verbose, far longer than the innerHTML option would be. This is when you appreciate having a JavaScript library that does this kind of thing for you. For example, in jQuery:

$('#y').html($('#x').clone(true, true).contents());

This is a lot more explicit about what you want to happen. As well as having various performance benefits and preserving event handlers, for example, it also helps you to understand what your code is doing. This is good for your soul as a JavaScript programmer and makes bizarre errors significantly less likely!

1. You can duplicate IDs which need to be unique.
2. jQuery's clone method call like, $(element).clone(true); will clone data and event listeners, but ID's will still also be cloned. So to avoid duplicate IDs, don't use IDs for items that need to be cloned.

1. It should be avoided because then you lose any handlers that may have been on that DOM element.

2. You can try to get around that by appending clones of the DOM elements instead of completely overwriting them.

First, let's define the task that has to be accomplished here:

All child nodes of DIV x have to be "copied" (together with all its descendants = deep copy) and "pasted" into the DIV y. If any of the descendants of x has one or more event handlers bound to it, we would presumably want those handlers to continue working on the copies (once they have been placed inside y).

Now, this is not a trivial task. Luckily, the jQuery library (and all the other popular libraries as well I assume) offers a convenient method to accomplish this task: .clone(). Using this method, the solution could be written like so:

$( x ).contents().clone( true ).appendTo( y );

The above solution is the answer to question (2). Now, let's tackle question (1):

This

y.innerHTML = x.innerHTML;

is not just a bad idea - it's an awful one. Let me explain...

The above statement can be broken down into two steps.

1. The expression x.innerHTML is evaluated,

2. That return value of that expression (which is a string) is assigned to y.innerHTML.

The nodes that we want to copy (the child nodes of x) are DOM nodes. They are objects that exist in the browser's memory. When evaluating x.innerHTML, the browser serializes (stringifies) those DOM nodes into a string (HTML source code string).

Now, if we needed such a string (to store it in a database, for instance), then this serialization would be understandable. However, we do not need such a string (at least not as an end-product).

In step 2, we are assigning this string to y.innerHTML. The browser evaluates this by parsing the string which results in a set of DOM nodes which are then inserted into DIV y (as child nodes).

So, to sum up:

Child nodes of x --> stringifying --> HTML source code string --> parsing --> Nodes (copies)

So, what's the problem with this approach? Well, DOM nodes may contain properties and functionality which cannot and therefore won't be serialized. The most important such functionality are event handlers that are bound to descendants of x - the copies of those elements won't have any event handlers bound to them. The handlers got lost in the process.

An interesting analogy can be made here:

Digital signal --> D/A conversion --> Analog signal --> A/D conversion --> Digital signal

As you probably know, the resulting digital signal is not an exact copy of the original digital signal - some information got lost in the process.

I hope you understand now why y.innerHTML = x.innerHTML should be avoided.