Uncaught TypeError Cannot set property 'top' of undefined

Question

I have a javascript file that is beaing loaded last. In the file this code is executed:

var x = document.getElementById("bildfram1").offsetHeight;

document.getElementsByTagName("header").style.top = "calc(x/2)";

But I get this error in the console:

Uncaught TypeError: Cannot set property 'top' of undefined

Any help on how to fix this! Thanks in advance!

Possible duplicate of [Javascript | Uncaught TypeError: Cannot set property 'color' of undefined](http://stackoverflow.com/questions/30232423/javascript-uncaught-typeerror-cannot-set-property-color-of-undefined) — Heretic Monkey, Jan 16 '17 at 17:45
Possible duplicate of [What do querySelectorAll, getElementsByClassName and other getElementsBy\* methods return?](http://stackoverflow.com/questions/10693845/what-do-queryselectorall-getelementsbyclassname-and-other-getelementsby-method) — Sebastian Simon, Jan 16 '17 at 17:48

zurfyx · Accepted Answer · 2017-01-16T17:48:17.367

getElementsByTagName returns an HTMLCollection of elements (an array-like object).

For this reason, you are required to target specific index(es).

document.getElementsByTagName('header')[0].style.top = "calc(x/2)"

calc is an experimental technology. If all you have to do a simple divison, you should probably do it beforehand.

var division = val/2;
document.getElementByTagName('header')[0].style.top = divison + 'px'

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