Why class attribute cannot assign inline by javascript?

use className, so change:

var div = document.getElementsByTagName("div");
div[0].class = "tagging";

var div = document.getElementsByTagName("div");
div[0].className = "tagging";

Demo:: jsFiddle

You need to use className.

Try:

div[0].className = "tagging";

If you want to add tha class to the existing one you can use:

div[0].className += " tagging"; // adding white-space is important

Demo here

To read: MDN className.

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