javascript - How do I add a class to a given element?

I have an element that already has a class:

Now I want to create a JavaScript function that will add a class to the div (not replace, but add).

How can I do that?

Answer

Add a space plus the name of your new class to the className property of the element. First, put an id on the element so you can easily get a reference.

Then

var d = document.getElementById("div1");
d.className += " otherclass";

Note the space before otherclass. It's important to include the space otherwise it compromises existing classes that come before it in the class list.

See also element.className on MDN.

Use element.classList.add to add a class:

element.classList.add("my-class");

And element.classList.remove to remove a class:

element.classList.remove("my-class");