Why doesn't getElementByClassName -> getElementsByTagName -> setAttribute work?

Question

I want open certain links in a new tab. Since I can't set it directly into the <a> tag, I want to put the link into <span> tags with a certain class name and set the target attribute via JavaScript.

I thought this would be easy, but I can't get it working:

addOnloadHook(function () {
  document.getElementByClassName('newTab').getElementsByTagName('a').setAttribute('target', '_blank');
});

<span class="newTab"><a href="http://www.com">Link</a></span>

What am I doing wrong?

Solution

document.getElementByClassName does not exist, the correct function is document.getElementsByClassName (note the extra s). It returns an array of matching nodes, so you've to give an index:

addOnloadHook(function () {
  document.getElementsByClassName('newTab')[0].getElementsByTagName('a')[0].setAttribute('target', '_blank');
});