Why is "element.innerHTML+=" bad code?

Question

I have been told not to append stuff using element.innerHTML += ... like this:

var str = "<div>hello world</div>";
var elm = document.getElementById("targetID");

elm.innerHTML += str; //not a good idea?

What is wrong with it?, what other alternatives do I have?

Answer 1

Every time innerHTML is set, the HTML has to be parsed, a DOM constructed, and inserted into the document. This takes time.

For example, if elm.innerHTML has thousands of divs, tables, lists, images, etc, then calling .innerHTML += ... is going to cause the parser to re-parse all that stuff over again. This could also break references to already constructed DOM elements and cause other chaos. In reality, all you want to do is append a single new element to the end.

It's better to just call appendChild:

var newElement = document.createElement('div');
newElement.innerHTML = '<div>Hello World!</div>';
elm.appendChild(newElement);​​​​​​​​​​​​​​​​

This way, the existing contents of elm are not parsed again.

NOTE: It's possible that [some] browsers are smart enough to optimize the += operator and not re-parse the existing contents. I have not researched this.