Concat() doesn't work

JavaScript
1

The following code does not work. Is it something wrong?

<html>
<body>

<div></div>
<div></div>
<div></div>
<span></span>
<span></span>


<script type="text/javascript">

var i = document.getElementsByTagName("div");
var j = document.getElementsByTagName("span");
i = i.concat(j) ;

alert(i.length);

</script>


</body>
</html>
2

I see. Thanks Raffles and pmw57! :slight_smile:

3

That’s because concat() is an Array method. i and j are not arrays, they are HTML Collections, which are objects that are “array-like” (you can loop through them, they have a length) but are not arrays (they don’t share many of the methods and properties arrays have).

If’ you really need them in a single collection, create an empty array and then loop through i and j, copying the items into your array.

4

You can also convert the array-like collection in to actual arrays by using splice, before performing the concat.

Something like the following should do the trick.


var i = document.getElementsByTagName("div");
i = Array.prototype.splice.call(i, 0, 0);

var j = document.getElementsByTagName("span");
j = Array.prototype.splice.call(j, 0, 0);

i = i.concat(j);