I'm trying to create an HTML element using JavaScript document.createElement("div") and then prepend/append it in my header of the document, but below is what I'm getting.

CodePudding user response：

You never defined header. You have to initialize it first by using var header = document.querySelector('header');

Then you can use header.prepend(...);

CodePudding user response：

Try to replace your header.prepend to:

document.getElementsByTagName("header")[0].prepend(message);