HTML getElementsByClassName returns HTMLCollection with length of 0

Using getElementsByClassName() will return all the elements with that class name in a document as a NodeList. This object represents a collection of nodes that can be accessed by index numbers, which starts in 0. In order to access the elements in the NodeList you will have to use a loop.

When you console.log(document.getElementsByClassName('gtableheader').length); you see 0 because when you run it there is no element with class gtableheader. You are able to see the items in the console because document.getElementsByClassName() returns a live collection that is updated when the new elements are added.

As well, in the code you are using and the length is 0, you can use the code below to access the class name.

document.getElementsByClassName('gtableheader')[0].style.color="red";

If you want to access all the elements in the class you can use a for loop.

var x = document.getElementsByClassName('gtableheader');
for (var i = 0; i < x.length; i++) {
    x[i].style.color = "red";
}

More information: http://www.w3schools.com/jsref/met_document_getelementsbyclassname.asp

That's because the getElementsByClassName returns a live collection. the length property of the object is 0 because at that point of time there is no element with that className in the DOM. Since the console shows the live representation of an object, it shows all the matching elements when the elements are added to the DOM.

DOM parser parses the documents from top to bottom, when it reaches to a tag, it parses it and adds the DOM representation of it (an instance of HTMLElement interface) to the Document Object Model. You should either move the script tag to the end of body tag or listen to DOMContentLoaded event which is fired when the initial HTML document has been completely loaded and parsed.